volume 2 2nd edition page 127.) But the offspring from a cross between

organisms which have been exposed to different conditions, profit in an incomparably higher degree than do young or old beings from a mere change in the conditions. In this latter case we never see anything like the effect which generally follows from a cross with another individual, especially from a cross with a fresh stock. This might, perhaps, have been expected, for the blending together of the sexual elements of two differentiated beings will affect the whole constitution at a very early period of life, whilst the organisation is highly flexible. We have, moreover, reason to believe that changed conditions generally act differently on the several parts or organs of the same individual (12/14. See, for instance, Brackenridge ‘Theory of Diathesis’ Edinburgh 1869.); and if we may further believe that these now slightly differentiated parts react on one another, the harmony between the beneficial effects on the individual due to changed conditions, and those due to the interaction of differentiated sexual elements, becomes still closer.

That wonderfully accurate observer, Sprengel, who first showed how important a part insects play in the fertilisation of flowers, called his book ‘The Secret of Nature Displayed;’ yet he only occasionally saw that the object for which so many curious and beautiful adaptations have been acquired, was the cross-fertilisation of distinct plants; and he knew nothing of the benefits which the offspring thus receive in growth, vigour, and fertility. But the veil of secrecy is as yet far from lifted; nor will it be, until we can say why it is beneficial that the sexual elements should be differentiated to a certain extent, and why, if the differentiation be carried still further, injury follows. It is an extraordinary fact that with many species, flowers fertilised with their own pollen are either absolutely or in some degree sterile; if fertilised with pollen from another flower on the same plant, they are sometimes, though rarely, a little more fertile; if fertilised with pollen from another individual or variety of the same species, they are fully fertile; but if with pollen from a distinct species, they are sterile in all possible degrees, until utter sterility is reached. We thus have a long series with absolute sterility at the two ends;--at one end due to the sexual elements not having been sufficiently differentiated, and at the other end to their having been differentiated in too great a degree, or in some peculiar manner.

The fertilisation of one of the higher plants depends, in the first place, on the mutual action of the pollen-grains and the stigmatic secretion or tissues, and afterwards on the mutual action of the contents of the pollen-grains and ovules. Both actions, judging from the increased fertility of the parent-plants and from the increased powers of growth in the offspring, are favoured by some degree of differentiation in the elements which interact and unite so as to form a new being. Here we have some analogy with chemical affinity or attraction, which comes into play only between atoms or molecules of a different nature. As Professor Miller remarks: “Generally speaking, the greater the difference in the properties of two bodies, the more intense is their tendency to mutual chemical action...But between bodies of a similar character the tendency to unite is feeble.” (12/15. ‘Elements of Chemistry’ 4th edition 1867 part 1 page 11. Dr. Frankland informs me that similar views with respect to chemical affinity are generally accepted by chemists.) This latter proposition accords well with the feeble effects of a plant’s own pollen on the fertility of the mother-plant and on the growth of the offspring; and the former proposition accords well with the powerful influence in both ways of pollen from an individual which has been differentiated by exposure to changed conditions, or by so-called spontaneous variation. But the analogy fails when we turn to the negative or weak effects of pollen from one species on a distinct species; for although some substances which are extremely dissimilar, for instance, carbon and chlorine, have a very feeble affinity for each other, yet it cannot be said that the weakness of the affinity depends in such cases on the extent to which the substances differ. It is not known why a certain amount of differentiation is necessary or favourable for the chemical affinity or union of two substances, any more than for the fertilisation or union of two organisms.

Mr. Herbert Spencer has discussed this whole subject at great length, and after stating that all the forces throughout nature tend towards an equilibrium, remarks, “that the need of this union of sperm-cell and germ-ccell is the need for overthrowing this equilibrium and re-establishing active molecular change in the detached germ--a result which is probably effected by mixing the slightly-different physiological units of slightly-different individuals.” (12/16. ‘Principles of Biology’ volume 1 page 274 1864. In my ‘Origin of Species’ published in 1859, I spoke of the good effects from slight changes in the condition of life and from cross-fertilisation, and of the evil effects from great changes in the conditions and from crossing widely distinct forms (i.e., species), as a series of facts “connected together by some common but unknown bond, which is essentially related to the principle of life.”) But we must not allow this highly generalised view, or the analogy of chemical affinity, to conceal from us our ignorance. We do not know what is the nature or degree of the differentiation in the sexual elements which is favourable for union, and what is injurious for union, as in the case of distinct species. We cannot say why the individuals of certain species profit greatly, and others very little by being crossed. There are some few species which have been self-fertilised for a vast number of generations, and yet are vigorous enough to compete successfully with a host of surrounding plants. We can form no conception why the advantage from a cross is sometimes directed exclusively to the vegetative system, and sometimes to the reproductive system, but commonly to both. It is equally inconceivable why some individuals of the same species should be sterile, whilst others are fully fertile with their own pollen; why a change of climate should either lessen or increase the sterility of self-sterile species; and why the individuals of some species should be even more fertile with pollen from a distinct species than with their own pollen. And so it is with many other facts, which are so obscure that we stand in awe before the mystery of life.

Under a practical point of view, agriculturists and horticulturists may learn something from the conclusions at which we have arrived. Firstly, we see that the injury from the close breeding of animals and from the self-fertilisation of plants, does not necessarily depend on any tendency to disease or weakness of constitution common to the related parents, and only indirectly on their relationship, in so far as they are apt to resemble each other in all respects, including their sexual nature. And, secondly, that the advantages of cross-fertilisation depend on the sexual elements of the parents having become in some degree differentiated by the exposure of their progenitors to different conditions, or from their having intercrossed with individuals thus exposed, or, lastly, from what we call in our ignorance spontaneous variation. He therefore who wishes to pair closely related animals ought to keep them under conditions as different as possible. Some few breeders, guided by their keen powers of observation, have acted on this principle, and have kept stocks of the same animals at two or more distant and differently situated farms. They have then coupled the individuals from these farms with excellent results. (12/17. ‘Variation of Animals and Plants under Domestication’ chapter 17 2nd edition volume 2 pages 98, 105.) This same plan is also unconsciously followed whenever the males, reared in one place, are let out for propagation to breeders in other places. As some kinds of plants suffer much more from self-fertilisation than do others, so it probably is with animals from too close interbreeding. The effects of close interbreeding on animals, judging again from plants, would be deterioration in general vigour, including fertility, with no necessary loss of excellence of form; and this seems to be the usual result.

It is a common practice with horticulturists to obtain seeds from another place having a very different soil, so as to avoid raising plants for a long succession of generations under the same conditions; but with all the species which freely intercross by aid of insects or the wind, it would be an incomparably better plan to obtain seeds of the required variety, which had been raised for some generations under as different conditions as possible, and sow them in alternate rows with seeds matured in the old garden. The two stocks would then intercross, with a thorough blending of their whole organisations, and with no loss of purity to the variety; and this would yield far more favourable results than a mere exchange of seeds. We have seen in my experiments how wonderfully the offspring profited in height, weight, hardiness, and fertility, by crosses of this kind. For instance, plants of Ipomoea thus crossed were to the intercrossed plants of the same stock, with which they grew in competition, as 100 to 78 in height, and as 100 to 51 in fertility; and plants of Eschscholtzia similarly compared were as 100 to 45 in fertility. In comparison with self-fertilised plants the results are still more striking; thus cabbages derived from a cross with a fresh stock were to the self-fertilised as 100 to 22 in weight.

Florists may learn from the four cases which have been fully described, that they have the power of fixing each fleeting variety of colour, if they will fertilise the flowers of the desired kind with their own pollen for half-a-dozen generations, and grow the seedlings under the same conditions. But a cross with any other individual of the same variety must be carefully prevented, as each has its own peculiar constitution. After a dozen generations of self-fertilisation, it is probable that the new variety would remain constant even if grown under somewhat different conditions; and there would no longer be any necessity to guard against intercrosses between the individuals of the same variety.

With respect to mankind, my son George has endeavoured to discover by a statistical investigation whether the marriages of first cousins are at all injurious, although this is a degree of relationship which would not be objected to in our domestic animals; and he has come to the conclusion from his own researches and those of Dr. Mitchell that the evidence as to any evil thus caused is conflicting, but on the whole points to its being very small. From the facts given in this volume we may infer that with mankind the marriages of nearly related persons, some of whose parents and ancestors had lived under very different conditions, would be much less injurious than that of persons who had always lived in the same place and followed the same habits of life. Nor can I see reason to doubt that the widely different habits of life of men and women in civilised nations, especially amongst the upper classes, would tend to counterbalance any evil from marriages between healthy and somewhat closely related persons.

Under a theoretical point of view it is some gain to science to know that numberless structures in hermaphrodite plants, and probably in hermaphrodite animals, are special adaptations for securing an occasional cross between two individuals; and that the advantages from such a cross depend altogether on the beings which are united, or their progenitors, having had their sexual elements somewhat differentiated, so that the embryo is benefited in the same manner as is a mature plant or animal by a slight change in its conditions of life, although in a much higher degree.

Another and more important result may be deduced from my observations. Eggs and seeds are highly serviceable as a means of dissemination, but we now know that fertile eggs can be produced without the aid of the male. There are also many other methods by which organisms can be propagated asexually. Why then have the two sexes been developed, and why do males exist which cannot themselves produce offspring? The answer lies, as I can hardly doubt, in the great good which is derived from the fusion of two somewhat differentiated individuals; and with the exception of the lowest organisms this is possible only by means of the sexual elements, these consisting of cells separated from the body, containing the germs of every part, and capable of being fused completely together.

It has been shown in the present volume that the offspring from the union of two distinct individuals, especially if their progenitors have been subjected to very different conditions, have an immense advantage in height, weight, constitutional vigour and fertility over the self-fertilised offspring from one of the same parents. And this fact is amply sufficient to account for the development of the sexual elements, that is, for the genesis of the two sexes.

It is a different question why the two sexes are sometimes combined in the same individual and are sometimes separated. As with many of the lowest plants and animals the conjugation of two individuals which are either quite similar or in some degree different, is a common phenomenon, it seems probable, as remarked in the last chapter, that the sexes were primordially separate. The individual which receives the contents of the other, may be called the female; and the other, which is often smaller and more locomotive, may be called the male; though these sexual names ought hardly to be applied as long as the whole contents of the two forms are blended into one. The object gained by the two sexes becoming united in the same hermaphrodite form probably is to allow of occasional or frequent self-fertilisation, so as to ensure the propagation of the species, more especially in the case of organisms affixed for life to the same spot. There does not seem to be any great difficulty in understanding how an organism, formed by the conjugation of two individuals which represented the two incipient sexes, might have given rise by budding first to a monoecious and then to an hermaphrodite form; and in the case of animals even without budding to an hermaphrodite form, for the bilateral structure of animals perhaps indicates that they were aboriginally formed by the fusion of two individuals.

It is a more difficult problem why some plants and apparently all the higher animals, after becoming hermaphrodites, have since had their sexes re-separated. This separation has been attributed by some naturalists to the advantages which follow from a division of physiological labour. The principle is intelligible when the same organ has to perform at the same time diverse functions; but it is not obvious why the male and female glands when placed in different parts of the same compound or simple individual, should not perform their functions equally well as when placed in two distinct individuals. In some instances the sexes may have been re-separated for the sake of preventing too frequent self-fertilisation; but this explanation does not seem probable, as the same end might have been gained by other and simpler means, for instance dichogamy. It may be that the production of the male and female reproductive elements and the maturation of the ovules was too great a strain and expenditure of vital force for a single individual to withstand, if endowed with a highly complex organisation; and that at the same time there was no need for all the individuals to produce young, and consequently that no injury, on the contrary, good resulted from half of them, or the males, failing to produce offspring.

There is another subject on which some light is thrown by the facts given in this volume, namely, hybridisation. It is notorious that when distinct species of plants are crossed, they produce with the rarest exceptions fewer seeds than the normal number. This unproductiveness varies in different species up to sterility so complete that not even an empty capsule is formed; and all experimentalists have found that it is much influenced by the conditions to which the crossed species are subjected. The pollen of each species is strongly prepotent over that of any other species, so that if a plant’s own pollen is placed on the stigma some time after foreign pollen has been applied to it, any effect from the latter is quite obliterated. It is also notorious that not only the parent species, but the hybrids raised from them are more or less sterile; and that their pollen is often in a more or less aborted condition. The degree of sterility of various hybrids does not always strictly correspond with the degree of difficulty in uniting the parent forms. When hybrids are capable of breeding inter se, their descendants are more or less sterile, and they often become still more sterile in the later generations; but then close interbreeding has hitherto been practised in all such cases. The more sterile hybrids are sometimes much dwarfed in stature, and have a feeble constitution. Other facts could be given, but these will suffice for us. Naturalists formerly attributed all these results to the difference between species being fundamentally distinct from that between the varieties of the same species; and this is still the verdict of some naturalists.

The results of my experiments in self-fertilising and cross-fertilising the individuals or the varieties of the same species, are strikingly analogous with those just given, though in a reversed manner. With the majority of species flowers fertilised with their own pollen yield fewer, sometimes much fewer seeds, than those fertilised with pollen from another individual or variety. Some self-fertilised flowers are absolutely sterile; but the degree of their sterility is largely determined by the conditions to which the parent plants have been exposed, as was well exemplified in the case of Eschscholtzia and Abutilon. The effects of pollen from the same plant are obliterated by the prepotent influence of pollen from another individual or variety, although the latter may have been placed on the stigma some hours afterwards. The offspring from self-fertilised flowers are themselves more or less sterile, sometimes highly sterile, and their pollen is sometimes in an imperfect condition; but I have not met with any case of complete sterility in self-fertilised seedlings, as is so common with hybrids. The degree of their sterility does not correspond with that of the parent-plants when first self-fertilised. The offspring of self-fertilised plants suffer in stature, weight, and constitutional vigour more frequently and in a greater degree than do the hybrid offspring of the greater number of crossed species. Decreased height is transmitted to the next generation, but I did not ascertain whether this applies to decreased fertility.

I have elsewhere shown that by uniting in various ways dimorphic or trimorphic heterostyled plants, which belong to the same undoubted species, we get another series of results exactly parallel with those from crossing distinct species. (12/18. ‘Journal of the Linnean Society Botany’ volume 10 1867 page 393.) Plants illegitimately fertilised with pollen from a distinct plant belonging to the same form, yield fewer, often much fewer seeds, than they do when legitimately fertilised with pollen from a plant belonging to a distinct form. They sometimes yield no seed, not even an empty capsule, like a species fertilised with pollen from a distinct genus. The degree of sterility is much affected by the conditions to which the plants have been subjected. (12/19. ‘Journal of the Linnean Society Botany’ volume 8 1864 page 180.) The pollen from a distinct form is strongly prepotent over that from the same form, although the former may have been placed on the stigma many hours afterwards. The offspring from a union between plants of the same form are more or less sterile, like hybrids, and have their pollen in a more or less aborted condition; and some of the seedlings are as barren and as dwarfed as the most barren hybrid. They also resemble hybrids in several other respects, which need not here be specified in detail,--such as their sterility not corresponding in degree with that of the parent plants,--the unequal sterility of the latter, when reciprocally united,--and the varying sterility of the seedlings raised from the same seed-capsule.

We thus have two grand classes of cases giving results which correspond in the most striking manner with those which follow from the crossing of so-called true and distinct species. With respect to the difference between seedlings raised from cross and self-fertilised flowers, there is good evidence that this depends altogether on whether the sexual elements of the parents have been sufficiently differentiated, by exposure to different conditions or by spontaneous variation. It is probable that nearly the same conclusion may be extended to heterostyled plants; but this is not the proper place for discussing the origin of the long-styled, short-styled and mid-styled forms, which all belong to the same species as certainly as do the two sexes of the same species. We have therefore no right to maintain that the sterility of species when first crossed and of their hybrid offspring, is determined by some cause fundamentally different from that which determines the sterility of the individuals both of ordinary and of heterostyled plants when united in various ways. Nevertheless, I am aware that it will take many years to remove this prejudice.

There is hardly anything more wonderful in nature than the sensitiveness of the sexual elements to external influences, and the delicacy of their affinities. We see this in slight changes in the conditions of life being favourable to the fertility and vigour of the parents, while certain other and not great changes cause them to be quite sterile without any apparent injury to their health. We see how sensitive the sexual elements of those plants must be, which are completely sterile with their own pollen, but are fertile with that of any other individual of the same species. Such plants become either more or less self-sterile if subjected to changed conditions, although the change may be far from great. The ovules of a heterostyled trimorphic plant are affected very differently by pollen from the three sets of stamens belonging to the same species. With ordinary plants the pollen of another variety or merely of another individual of the same variety is often strongly prepotent over its own pollen, when both are placed at the same time on the same stigma. In those great families of plants containing many thousand allied species, the stigma of each distinguishes with unerring certainty its own pollen from that of every other species.

There can be no doubt that the sterility of distinct species when first crossed, and of their hybrid offspring, depends exclusively on the nature or affinities of their sexual elements. We see this in the want of any close correspondence between the degree of sterility and the amount of external difference in the species which are crossed; and still more clearly in the wide difference in the results of crossing reciprocally the same two species;--that is, when species A is crossed with pollen from B, and then B is crossed with pollen from A. Bearing in mind what has just been said on the extreme sensitiveness and delicate affinities of the reproductive system, why should we feel any surprise at the sexual elements of those forms, which we call species, having been differentiated in such a manner that they are incapable or only feebly capable of acting on one another? We know that species have generally lived under the same conditions, and have retained their own proper characters, for a much longer period than varieties. Long-continued domestication eliminates, as I have shown in my ‘Variation under Domestication,’ the mutual sterility which distinct species lately taken from a state of nature almost always exhibit when intercrossed; and we can thus understand the fact that the most different domestic races of animals are not mutually sterile. But whether this holds good with cultivated varieties of plants is not known, though some facts indicate that it does. The elimination of sterility through long-continued domestication may probably be attributed to the varying conditions to which our domestic animals have been subjected; and no doubt it is owing to this same cause that they withstand great and sudden changes in their conditions of life with far less loss of fertility than do natural species. From these several considerations it appears probable that the difference in the affinities of the sexual elements of distinct species, on which their mutual incapacity for breeding together depends, is caused by their having been habituated for a very long period each to its own conditions, and to the sexual elements having thus acquired firmly fixed affinities. However this may be, with the two great classes of cases before us, namely, those relating to the self-fertilisation and cross-fertilisation of the individuals of the same species, and those relating to the illegitimate and legitimate unions of heterostyled plants, it is quite unjustifiable to assume that the sterility of species when first crossed and of their hybrid offspring, indicates that they differ in some fundamental manner from the varieties or individuals of the same species.

INDEX.

Abutilon darwinii, self-sterile in Brazil. moderately self-fertile in England. fertilised by birds.

Acacia sphaerocephala.

Acanthaceae.

Aconitum napellus.

Adlumia cirrhosa.

Adonis aestivalis. measurements. relative heights of crossed and self-fertilised plants. self-fertile.

Ajuga reptans.

Allium cepa (blood-red var.)

Anagallis collina (var. grandiflora). measurements. seeds.

Anderson, J., on the Calceolaria. removing the corollas.

Anemone.

Anemophilous plants. often diclinous.

Antirrhinum majus (red var.) perforated corolla. --(white var.). --(peloric var.).

Apium petroselinum. result of experiments.

Argemone ochroleuca.

Aristolochia.

Aristotle on bees frequenting flowers of the same species.

Arum maculatum.

Bailey, Mr., perforation of corolla.

Bartonia aurea. measurements. result of experiments.

Bartsia odontites.

Beal, W.J., sterility of Kalmia latifolia. on nectar in Ribes aureum.

Bean, the common.

Bees distinguish colours. frequent the flowers of the same species. guided by coloured corolla. powers of vision and discrimination. memory. unattracted by odour of certain flowers. industry. profit by the corolla perforated by humble-bees. skill in working. habit. foresight.

Bees, humble, recognise varieties as of one species. colour not the sole guide. rate of flying. number of flowers visited. corolla perforated by. skill and judgment.

Belt, Mr., the hairs of Digitalis purpurea. Phaseolus multiflorus. not visited by bees in Nicaragua. humming-birds carrying pollen. secretion of nectar. in Acacia sphaerocephalus and passion-flower. perforation of corolla.

Bennett, A.W., on Viola tricolor. structure of Impatiens fulva. plants flowering in winter. bees frequenting flowers of same species.

Bentham, on protection of the stigma in Synaphea.

Beta vulgaris. measurements. crossed not exceeded by self-fertilised. prepotency of other pollen.

Bignonia.

Birds, means of fertilisation.

Blackley, Mr., on anthers of rye. pollen carried by wind, experiments with a kite.

Boraginaceae.

Borago officinalis. measurements. early flowering of crossed. seeds. partially self-sterile.

Brackenridge, Mr., organism of animals affected by temperature and food. different effect of changed conditions.

Brassica oleracea. measurements. weight. remarks on experiments. superiority of crossed. period of flowering. seeds. self-fertile. --napus. --rapa.

Brisout, M., insects frequenting flowers of same species.

Broom.

Brugmansia. humming-birds boring the flower.

Bulrush, weight of pollen produced by one plant.

Bundy, Mr., Ribes perforated by bees.

Butschli, O., sexual relations.

Cabbage. affected by pollen of purple bastard. prepotency of other pollen. --, Ragged Jack.

Calceolaria.

Calluna vulgaris.

Campanula carpathica.

Campanulaceae.

Candolle, A. de, on ascending a mountain the flowers of the same species disappear abruptly.

Canna warscewiczi. result of crossed and self-fertilised. period of flowering. seeds. highly self-fertile.

Cannaceae.

Carduus arctioides.

Carnation.

Carriere, relative period of the maturity of the sexual elements on same flower.

Caryophyllaceae.

Caspary, Professor, on Corydalis cava. Nymphaeaceae. Euryale ferox.

Cecropia, food-bodies of.

Centradenia floribunda.

Cereals, grains of.

Cheeseman, Mr., on Orchids in New Zealand.

Chenopodiaceae.

Cineraria.

Clarkia elegans. measurements. early flowering of self-fertilised. seeds.

Cleistogene flowers.

Coe, Mr., crossing Phaseolus vulgaris.

Colgate, R., red clover never sucked by hive-bees in New Zealand.

Colour, uniform, of flowers on plants self-fertilised and grown under similar conditions for several generations.

Colours of flowers attractive to insects. not the sole guide to bees.

Compositae.

Coniferae.

Convolvulus major. -- tricolor.

Corolla, removal of. perforation by bees.

Coronilla.

Corydalis cava. -- halleri. -- intermedia. -- lutea. -- ochroleuca. -- solida.

Corylus avellana.

Cowslip.

Crinum.

Crossed plants, greater constitutional vigour of.

Cross-fertilisation. see Fertilisation.

Crossing flowers on same plant, effects of.

Cruciferae.

Cruger, Dr., secretion of sweet fluid in Marcgraviaceae.

Cuphea purpurea.

Cycadiae.

Cyclamen persicum. measurements. early flowering of crossed. seeds. self-sterile. -- repandum.

Cytisus laburnum.

Dandelion, number of pollen grains.

Darwin, C., self-fertilisation in Pisum sativum. sexual affinities. on Primula. bud variation. constitutional vigour from cross parentage in common pea. hybrids of Gladiolus and Cistus. Phaseolus multiflorus. nectar in Orchids. on cross-fertilisation. inheritance of acquired modifications. change in the conditions of life beneficial to plants and animals.

Darwin, F., structure of Phaseolus multiflorus. Pteris aquilina. perforation of Lathyrus sylvestris.

Darwin, G., on marriages with first cousins.

Decaisne on Delphinium consolida.

De Candolle, nectar as an excretion.

Delphinium consolida. measurements. seeds. partially sterile. corolla removed.

Delpino, Professor, Viola tricolor. Phaseolus multiflorus. intercrossing of sweet-pea. Lobelia ramosa. structure of the Cannaceae. wind and water carrying pollen. Juglans regia. anemophilous plants. fertilisation of Plantago. excretion of nectar. secretion of nectar to defend the plant. anemophilous and entomophilous plants. dioecious plants.

Denny, Pelargonium zonale.

Diagram showing mean height of Ipomoea purpurea.

Dianthus caryophyllus. crossed and self-fertilised. measurements. cross with fresh stock. weight of seed. colour of flowers. remarks on experiments. early flowering of crossed. uniform colour of self-fertilised. seeds. few capsules.

Dickie, Dr., self-fertilisation in Cannaceae.

Dictamnus fraxinella.

Digitalis purpurea. measurements. effects of intercrossing. superiority of crossed. self-sterile.

Dipsaceae.

Dobbs, bees frequenting flowers of same species.

Dodel, Dr. A., sexual reproduction.

Duhamel on Raphanus sativus.

Dunal, nectar as an excretion.

Dyer, Mr., on Lobelia ramosa. on Cineraria.

Earley, W., self-fertilisation of Lathyrus odoratus.

Eaton, Reverend A.E., on Pringlea.

Engelmann, development of sexual forms.

Engler, Dr., on dichogamous Saxifraga.

Entomophilous plants.

Epipactis latifolia, attractive only to wasps.

Erica tetralix. perforated corolla.

Erythrina.

Eschscholtzia californica. measurements. plants raised from Brazilian seed. weight. seeds. experiments on. superiority of self-fertilised over crossed. early flowering. artificially self-fertilised. pollen from other flowers more effective. self-sterile in Brazil.

Euphrasia officinalis.

Euryale amazonica. -- ferox.

Fabricius on Aristolochia.

Fagopyrum esculentum. early flowering of crossed plant.

Faivre, Professor, self-fertilisation of Cannaceae.

Farrer, T.H., papilionaceous flowers. Lupinus luteus. Phaseolus multiflorus. Pisum sativum. cross-fertilisation of Lobelia ramosa. on Coronilla.

Fermond, M., Phaseolus multiflorus. Phaseolus coccineus hybridus.

Fertilisation, means of. plants sterile, or partially so without insect-aid. plants fertile without insect-aid. means of cross-fertilisation. humming-birds. Australian flowers fertilised by honey-sucking birds. in New Zealand by the Anthornis melanura. attraction of bright colours. of odours. flowers adapted to certain kinds of insects. large amount of pollen-grains. transport of pollen by insects. structure and conspicuousness of flowers. pollen from a distinct plant. prepotent pollen.

Fertility, heights and weights, relative, of plants crossed by a fresh stock, self-fertilised, or intercrossed (Table 7/C).

Fertility of plants as influenced by cross and self-fertilisation (Table 9/D). relative, of crossed and self-fertilised parents (Table 9/E). innate, from a cross with fresh stock (Table 9/F). relative, of flowers crossed with pollen from a distinct plant and their own pollen (Table 9/G). of crossed and self-fertilised flowers.

Flowering, period of, superiority of crossed over self-fertilised.

Flowers, white, larger proportion smelling sweetly. structure and conspicuousness of. conspicuous and inconspicuous. papilionaceous. fertilised with pollen from a distinct plant.

Forsythia viridissima.

Foxglove. Frankland, Dr., chemical affinity.

Fraxinus ornus.

Fumaria capreolata. -- officinalis.

Galium aparine.

Gallesio, spontaneous crossing of oranges.

Galton, Mr., Limnanthes douglasii. report on the tables of measurements. self-fertilised plants. superior vigour of crossed seedlings in Lathyrus odoratus.

Gartner, excess of pollen injurious. plants fertilising one another at a considerable distance. Lobelia fulgens. sterility of Verbascum nigrum. number of pollen-grains to fertilise Geum urbanum. experiments with pollen.

Gentry, Mr., perforation of corolla.

Geraniaceae.

Geranium phaeum.

Gerardia pedicularia.

Germination, period of, and relative weight of seeds from crossed and self-fertilised flowers.

Gesneria pendulina. measurements. seeds.

Gesneriaceae.

Geum urbanum, number of pollen-grains for fertilisation.

Glaucium luteum.

Godron, intercrossing of carrot. Primula grandiflora affected by pollen of Primula officinalis. tulips.

Gould, humming-birds frequenting Impatiens.

Graminaceae.

Grant, Mr., bees of different hives visiting different kinds of flowers.

Gray, Asa, sexual relations of trees in United States. on sexual reproduction.

Hallet, Major, on selection of grains of cereals.

Hassall, Mr., number of pollen-grains in Paeony and Dandelion. weight of pollen produced by one plant of Bulrush.

Heartsease.

Hedychium.

Hedysarum onobrychis.

Heights, relative, of crossed and self-fertilised plants (Table 7/A).

Heights, weights, and fertility, summary.

Henschel’s experiments with pollen.

Henslow, Reverend G., cross-fertilisation in Sarothamnus scoparius.

Herbert on cross-fertilisation. pollen brought from distant plants. spontaneous crossing of rhododendrons.

Hero, descendants of the plant. its self-fertilisation.

Heterocentron mexicanum.

Hibiscus africanus. measurements. result of experiments. early flowering of crossed plant. number of pollen-grains for fertilisation.

Hildebrand on pollen of Digitalis purpurea. Thunbergia alata. experiments on Eschscholtzia californica. Viola tricolor. Lobelia ramosa. Fagopyrum esculentum. self-fertilisation of Zea mays. Corydalis cava. Hypecoum grandiflorum. and Hypecoum procumbens. sterility of Eschscholtzia. experiments on self-fertilisation. Corydalis lutea. spontaneously self-fertilised flowers. various mechanical structure to check self-fertilisation. early separation of the sexes. on Aristolochia. fertilisation of the Gramineae. wide dissemination of seeds.

Hoffmann, Professor H., self-fertilised capsules of Papaver somniferum. Adonis aestivalis. spontaneous variability of Phaseolus multiflorus. self-fertilisation of kidney-bean. Papaver alpinum. sterility of Corydalis solida. Linum usitatissimum.

Honey-dew.

Hooker, Dr., Euryale ferox and Victoria regia, each producing several flowers at once. on sexual relation of trees in New Zealand.

Horse-chestnut.

Humble-bees, see Bees.

Humboldt, on the grains of cereals.

Humming-Birds a means of cross-fertilisation.

Hyacinth.

Hybrid plants, tendency to revert to their parent forms.

Hypecoum grandiflorum. -- procumbens.

Iberis umbellata (var. kermesiana). measurement. cross by fresh stocks. remarks on experiments. superiority of crossed over self-fertilised seedlings. early flowering. number of seeds. highly self-fertile. prepotency of other pollen. -- amara.

Impatiens frequented by humming-birds. -- barbigera. -- fulva. -- noli-me-tangere. -- pallida.

Inheritance, force of, in plants.

Insects, means of cross-fertilisation. attracted by bright colours. by odours. by conspicuous flowers. dark streaks and marks as guides for. flowers adapted to certain kinds.

Ipomoea purpurea. measurements. flowers on same plant crossed. cross with fresh stock. descendants of Hero. summary of measurements. diagram showing mean heights. summary of observations. of experiments. superiority of crossed. early flowering. effects of intercrossing. uniform colour of self-fertilised. seeds. highly self-fertile. prepotency of other pollen.

Iris, secretion of saccharine matter from calyx.

Isotoma.

Juglans regia.

Kalmia latifolia.

Kerner, on protection of the pollen. on the single daily flower of Villarsia parnassifolia. pollen carried by wind.

Kidney-bean.

Kitchener, Mr., on the action of the stigma. on Viola tricolor.

Knight, A., on the sexual intercourse of plants. crossing varieties of peas. sexual reproduction.

Kohl-rabi, prepotency of pollen.

Kolreuter on cross-fertilisation. number of pollen-grains necessary for fertilisation. sexual affinities of Nicotiana. Verbascum phoeniceum. experiments with pollen of Hibiscus vesicarius.

Kuhn adopts the term cleistogene.

Kurr, on excretion of nectar. removal of corolla.

Labiatae.

Lactuca sativa. measurement. prepotency of other pollen.

Lamium album. -- purpureum.

Lathyrus odoratus. measurements. remarks on experiments. period of flowering. cross-fertilisation. seeds. self-fertile. -- grandiflorus. -- nissolia. -- sylvestris, perforation of corolla.

Lawes and Gilbert, Messrs., consumption of inorganic matter by plants.

Laxton, Mr., crossing varieties of peas.

Lecoq, Cyclamen repandum. on Fumariaceae. annual plants rarely dioecious.

Leersia oryzoides.

Leguminosae. summary on the.

Leighton, Reverend W.A., on Phaseolus multiflorus. Acacia magnifica.

Leptosiphon androsaceus.

Leschenaultia formosa.

Lettuce.

Lilium auratum.

Limnanthes douglasii. measurements. early flowering of crossed. seeds. highly self-fertile. prepotency of other pollen.

Linaria vulgaris. seeds. self-sterile. -- cymbalaria.

Lindley on Fumariaceae.

Link, hypopetalous nectary in Chironia decussata.

Linum grandiflorum. -- usitatissimum.

Loasaceae.

Lobelia erinus. secretion of nectar in sunshine. experiments with bees.

Lobelia fulgens. measurements. summary of experiments. early flowering of self-fertilised. seeds. sterile unless visited by humble-bees. -- ramosa. measurements. early flowering of crossed. seeds. self-sterile. -- tenuior.

Loiseleur-Deslongchamp, on the grains of cereals.

Lotus corniculatus.

Lubbock, Sir J., cross-fertilisation of flowers. on Viola tricolor. bees distinguishing colours. instinct of bees and insects sucking nectar.

Lupinus luteus. measurements. early flowering of self-fertilised. self-fertile. prepotency of other pollen. -- pilosus. self-fertile.

Lychnis dioica.

MacNab, Mr., on the shorter or longer stamens of rhododendrons.

Mahonia aquifolium. -- repens.

Malvaceae.

Marcgraviaceae.

Masters, Mr., cross-fertilisation in Pisum sativum. cabbages affected by pollen at a distance.

Masters, Dr. Maxwell, on honey-dew.

Measurements, summary of. Table 7/A. Table 7/B. Table 7/C.

Medicago lupulina.

Meehan, Mr., fertilising Petunia violacea by night moth.

Melastomaceae.

Melilotus officinalis.

Mercurialis annua.

Miller, Professor, on chemical affinity.

Mimulus luteus, effects of crossing. crossed and self-fertilised plants. measurements. cross with a distinct stock. intercrossed on same plant. summary of observations. of experiments. superiority of crossed plants. simultaneous flowering. effects of intercrossing. uniform colour of self-fertilised. seeds. highly self-fertile. prepotency of other pollen. -- roseus.

Miner, Mr., red clover never sucked by hive-bees in the United States.

Mirabilis, dwarfed plants raised by using too few pollen-grains. number of grains necessary for fertilisation.

Mitchell, Dr., on first cousins inter-marrying.

Monochaetum ensiferum.

Moore, Mr., on Cinerarias.

Muller, Fritz, on Posoqueria fragrans. experiments on hybrid Abutilons and Bignonias. large number of Orchidaceous genera sterile in their native home, also Bignonia and Tabernaemontana echinata. sterility of Eschscholtzia californica. Abutilon darwinii. experiments in self-fertilisation. self-sterile plants. incapacity of pollen-tubes to penetrate the stigma. cross-fertilisation by means of birds. imperfectly developed male and female Termites. food-bodies in Cecropia.

Muller, Hermann, fertilisation of flowers by insects. on Digitalis purpurea. Calceolaria. Linaria vulgaris. Verbascum nigrum. the common cabbage. Papaver dubium. Viola tricolor. structure of Delphinium consolida. of Lupinus lutea. flowers of Pisum sativum. on Sarothamnus scoparius not secreting nectar. Apium petroselinum. Borago officinalis. red clover visited by hive-bees in Germany. insects rarely visiting Fumaria officinalis. comparison of lowland and alpine species. structure of plants adapted to cross and self-fertilisation. large conspicuous flowers more frequently visited by insects than small inconspicuous ones. Solanum generally unattractive to insects. Lamium album. on anemophilous plants. fertilisation of Plantago. secretion of nectar. instinct of bees sucking nectar. bees frequenting flowers of the same species. cause of it. powers of vision and discrimination of bees.

Muller, Dr. H., hive-bees occasionally perforate the flower of Erica tetralix. calyx and corolla of Rhinanthus alecterolophus bored by Bombus mastrucatus.

Munro, Mr., some species of Oncidium and Maxillaria sterile with own pollen.

Myrtaceae.

Nageli on odours attracting insects. sexual relations.

Natural selection, effect upon self-sterility and self-fertilisation.

Naudin on number of pollen-grains necessary for fertilisation. Petunia violacea.

Nectar regarded as an excretion.

Nemophila insignis. measurements. early flowering of crossed plant. effects of cross and self-fertilisation. seeds.

Nepeta glechoma.

Nicotiana glutinosa. -- tabacum. measurements. cross with fresh stock. measurements. summary of experiments. superiority of crossed plants. early flowering. seeds. experiments on. self-fertile.

Nolana prostrata. measurements. crossed and self-fertilised plants. number of capsules and seeds. self-fertile.

Nolanaceae.

Nymphaea.

Odours emitted by flowers attractive to insects.

Ogle, Dr., on Digitalis purpurea. Gesneria. Phaseolus multiflorus. perforation of corolla. case of the Monkshood.

Onagraceae.

Onion, prepotency of other pollen.

Ononis minutissima. measurements. seeds. self-fertile.

Ophrys apifera. -- muscifera.

Oranges, spontaneous crossing.

Orchideae. excretion of saccharine matter.

Orchis, fly.

Origanum vulgare. measurements. early flowering of crossed plant. effects of intercrossing.

Paeony, number of pollen-grains.

Papaveraceae.

Papaver alpinum. -- argemonoides. -- bracteatum. -- dubium. -- orientale. -- rhoeas. -- somniferum. -- vagum. measurements. number of capsules. seeds. prepotency of other pollen.

Papillae of the Viola tricolor attractive to insects.

Parsley.

Passiflora alata. -- gracilis. measurements. crossed and self-fertilised. seeds. self-fertile.

Passifloraceae.

Pea, common.

Pelargonium zonale. measurements. effects of intercrossing. almost self-sterile.

Pentstemon argutus, perforated corolla.

Petunia violacea. measurements. weight of seed. cross with fresh stock. relative fertility. colour. summary of experiments. superiority of crossed over self-fertilised. early flowering. uniform colour of self-fertilised. seeds. self-sterile.

Phalaris canariensis. measurements. early flowering of crossed.

Phaseolus coccineus. -- multiflorus. measurement. partially sterile. crossed and self-fertilised. early flowering of crossed. seeds. perforated by humble-bees. -- vulgaris. self-fertile.

Pisum sativum. measurements. seldom intercross. summary of experiments. self-fertile.

Plants, crossed, greater constitutional vigour.

Pleroma.

Polemoniaceae.

Pollen, relative fertility of flowers crossed from a distinct plant, or with their own. difference of results in Nolana prostrata. crossed and self-fertilised plants, again crossed from a distinct plant and their own pollen. sterile with their own. semi-self-sterile. loss of. number of grains in Dandelion, Paeony, and Wistaria sinensis. number necessary for fertilisation. transported from flower to flower. prepotency. aboriginally the sole attraction to insects. quantity produced by anemophilous plants.

Polyanthus, prepotency over cowslip.

Polygoneae.

Posoqueria fragrans.

Potato.

Poterium sanguisorba.

Potts, heads of Anthornis melanura covered with pollen.

Primrose, Chinese.

Primula elatior. -- grandiflora. -- mollis. -- officinalis. -- scotica. -- sinensis. measurements. early flowering of crossed. -- veris (var. officinalis). measurements. result of experiments. early flowering of crossed. seeds. self-fertility. prepotency of dark red polyanthus.

Primulaceae.

Pringlea.

Proteaceae of Australia.

Prunus avium. -- laurocerasus.

Pteris aquilina.

Radish.

Ranunculaceae.

Ranunculus acris.

Raphanus sativus.

Reinke, nectar-secreting glands of Prunus avium.

Reseda lutea. measurements. result of experiments. self-fertile. -- odorata. measurements. self-fertilised scarcely exceeded by crossed. seeds. want of correspondence between seeds and vigour of offspring. result of experiments. sterile and self-fertile.

Resedaceae.

Rheum rhaponticum.

Rhexia glandulosa.

Rhododendron, spontaneous crossing.

Rhododendron azaloides.

Rhubarb.

Ribes aureum.

Riley, Mr., pollen carried by wind. Yucca moth.

Rodgers, Mr., secretion of nectar in Vanilla.

Rye, experiment on pollen of.

Salvia coccinea. measurements. early flowering of crossed. seeds. partially self-sterile. -- glutinosa. -- grahami. -- tenori.

Sarothamnus scoparius. measurements. superiority of crossed seedlings. seeds. self-sterile.

Scabiosa atro-purpurea. measurements.

Scarlet-runner.

Scott, J., Papaver somniferum. sterility of Verbascum. Oncidium and Maxillaria. on Primula scotica and Cortusa matthioli.

Scrophulariaceae.

Self-sterile varieties, appearance of.

Self-fertilisation, mechanical structure to check.

Self-sterile plants. wide distribution throughout the vegetable kingdom. difference in plants. cause of self-sterility. affected by changed conditions. necessity of differentiation in the sexual elements.

Senecio cruentus. -- heritieri. -- maderensis -- populifolius. -- tussilaginis.

Sharpe, Messrs., precautions against intercrossing.

Snow-flake.

Solanaceae.

Solanum tuberosum.

Specularia perfoliata. -- speculum. measurements. crossed and self-fertilised. early flowering of crossed. seeds. self-fertile.

Spencer, Herbert, chemical affinity.

Spiranthes autumnalis.

Sprengel, C.K., fertilisation of flowers by insects. Viola tricolor. colours in flowers attract and guide insects. on Aristolochia. Aconitum napellus. importance of insects in fertilising flowers.

Stachys coccinea.

Stellaria media.

Strachey, General, perforated flowers in the Himalaya.

Strawberry.

Strelitzia fertilised by the Nectarinideae.

Structure of plants adapted to cross and self-fertilisation.

Swale, Mr., garden lupine not visited by bees in New Zealand.

Sweet-pea.

Tabernaemontana echinata.

Tables of measurements of heights, weights, and fertility of plants.

Termites, imperfectly developed males and females.

Thunbergia alata.

Thyme.

Tinzmann, on Solanum tuberosum.

Tobacco.

Transmission of the good effects of a cross to later generations.

Trees, separated sexes.

Trifolium arvense. -- incarnatum. -- minus. -- pratense. -- procumbens. -- repens.

Tropaeolum minus. measurements. early flowering of crossed. seeds. -- tricolor. seeds.

Tulips.

Typha.

Umbelliferae.

Urban, Ig., fertilisation of Medicago lupulina.

Vandellia nummularifolia. seeds. self-fertile.

Vanilla, secretion of nectar.

Verbascum lychnitis. -- nigrum. -- phoeniceum. -- thapsus. measurements. self-fertile.

Verlot on Convolvulus tricolor. intercrossing of Nemophila. of Leptosiphon.

Veronica agrestis. -- chamaedrys. -- hederaefolia.

Vicia faba. -- hirsuta. -- sativa.

Victoria regia.

Villarsia parnassifolia.

Vilmorin on transmitting character to offspring.

Vinca major. -- rosea.

Viola canina. -- tricolor. measurements. superiority of crossed plants. period of flowering. effects of cross-fertilisation. seeds. partially sterile. corolla removed.

Violaceae.

Viscaria oculata. measurement. average height of crossed and self-fertilised. simultaneous flowering. seeds. self-fertile.

Wallace, Mr., the beaks and faces of brush-tongued lories covered with pollen.

Wasps attracted by Epipactis latifolia.

Weights, relative, of crossed and self-fertilised plants. and period of germination of seeds.

Wilder, Mr., fertilisation of flowers with their own pollen.

Wilson, A.J., superior vigour of crossed seedlings in Brassica campestris ruta baga.

Wistaria sinensis.

Yucca moth.

Zea mays. measurements. difference of height between crossed and self-fertilised. early flowering of crossed. self-fertile. prepotency of other pollen.