Part 44

The germ-plasm of the ovum which is ripe for development must thus contain not only the determinants of the specific ova and sperms of the species, but also those of all the male and female sexual characters, which we discussed at length in the section on sexual selection. I then showed that these secondary sexual characters differ greatly in range and in strength, that among lower animals they are almost entirely absent, and that among higher forms, such Crustaceans, Insects, and Birds, they attain to very different grades of development even among the same species. Thus the birds of Paradise are in most species brilliantly coloured and adorned with decorative feathers only in the male sex, while the females are simply blackish-grey, but there is a single species in which the males are almost as soberly coloured as the females. Conversely, too, we find that in parrots both sexes are usually coloured alike, but a few species exhibit a totally different colouring in the two sexes. In the same way the secondary sex differences may affect only a few parts of the animal or many, while in a few species the sexes are so divergent in structure that almost everything about them may be called different. Examples of this are the dwarf males of most Rotifers, and the males, more minute still in proportion to the females, of the marine worm _Bonellia viridis_ (p. 227).

We have now to inquire what theoretical explanation of these facts we can arrive at in accordance with the germ-plasm theory. That double determinants, male and female, for the differently formed parts of the two sexes must be assumed to exist in the germ-plasm has been already said, and we have to suppose that the same stimulus--usually unknown to us--which incites the determinants of the primary sexual characters to activity also liberates those of the secondary characters. But we may safely go a step further and conclude that there are male and female _ids_, that is, that the male and female determinants belong to different ids. I infer this from the fact that in some groups, such as the Rotifers and certain plant-lice, the ova are sexually differentiated even at the time of their origin. Males and females of these animals arise from different kinds of eggs, which are even externally recognizable. Both develop parthenogenetically, so that fertilization has nothing to do with it; from the first, therefore, they must contain ids which consist of determinants of one sex alone.

If this conclusion be correct, then the sexual equipment of the determinants of the sexual characters must have taken place in the course of phylogeny in such a way that each id was affected in one direction only, and we should thus have to assume male and female ids, even before the separation of the sexes as males and females, and the same conclusion must be extended to the primary sexual characters. Only in this way can we understand the fact that differences between the sexes, at first small, have increased in the course of phylogeny to such complete divergence of structure as is now exhibited in the forms we have named, _Bonellia_, the Rotifers, and some parasitic worms.

But there is not only sexual dimorphism, there is also dimorphism of larvæ, e.g. green and brown caterpillars in certain species of hawk-moth (_Sphinx_), and there are sometimes not only two but three or more forms of a species; and in all these cases determinants of the differential parts must be represented twice, thrice, or several times in each germ-plasm, in each fertilized ovum, at least in all cases in which the different forms live together on the same area. In discussing mimicry we spoke of species of butterfly which were everywhere alike or nearly so in the male sex, while the females were not only quite different from the males, but differed greatly in many respects among themselves. Three different forms of females of _Papilio merope_ occur in the same region of Cape Colony, each of these resembling a protected model. All three forms have been obtained from the eggs of one female. In this case the female ids of the germ-plasm must be represented by three different sets, one of which, when it is in the majority in the fertilized ovum, gives rise to the _Danais_-form, the second to the _Niavius_-form, and the third to the _Echeria_-form of the species. Phylogenetically considered, it is probable that each of these three kinds of ids originated by itself, on a more limited area on which the protected model lived in abundance; but with a wider distribution the different female ids mingled together, were united through the males into a single germ-plasm, and now occasionally exhibit all three forms on the same area. I doubt whether there is any other theory that can offer an interpretation of these facts, and I regard them, therefore, as affording further evidence of the real existence of ids.

The polymorphism of social insects must be thought of as similarly based in the germ-plasm.

In bees there are in addition to the males and females the so-called workers, and this can only depend on the existence of special kinds of ids. Those of the workers were originally truly female, but as many of their determinants underwent variations advantageous for the maintenance of the species, they were modified into special 'worker-ids.' I postpone for the present any inquiry into the causes by which these ids come to dominate the ontogeny; obviously it cannot be by the mere fact of being in a majority over the rest of the ids, as I indicated in the case of the butterflies with polymorphic females.

In many ants the division of labour goes further still; there are two kinds of workers in the colony, the ordinary workers and the so-called 'soldiers,' and in this case the worker-id must have developed in two different directions in the course of phylogeny, and have separated into two kinds of ids, so that the germ-plasm of these species must contain four kinds of ids.

I might cite many more cases in regard to which the assumption of two or more kinds of determinants seems imperative, but I believe that what has been said is enough to enable any one to think out other cases for himself.

LECTURE XIX

THE GERM-PLASM THEORY (_continued_)

Co-operation of the determinants to form an organ: insect appendages--Venation of the insect-wing--Deformities in Man--Apex of the fly's leg--Proofs of the existence of determinants--Claws and adhesive lobes--Difference between a theory of development and a theory of heredity--Metamorphosis of the food-canal in insects--Delage's theory--Reinke's theory of the organism-machine--Fechner's views--Apparent contradiction by the facts of developmental mechanics--Formation of the germ-cells--Displacement of the germinal areas in the hydro-medusoid polyps, a proof of the existence of germ-tracks.

IT would be futile to attempt to guess at the arrangement of the determinants in the germ-plasm, but so much at least we may say, that the determinants do not lie beside each other in the same disposition as their determinates exhibit in the fully-formed organism. This may be inferred from the complex formative processes of embryogenesis in which many groups of cells, which in their origin were far apart, combine together to form an organ. Thus the arrangement of the determinants in the germ-plasm does not correspond to the subsequent arrangement of the whole animal, nor are primary constituents of the _complete_ organs contained within the germ-plasm. The organ is undoubtedly _predetermined_ in the germ-plasm, but it is not _preformed_ as such.

Here, again, the history of development gives us a certain basis of fact from which to work. Let us consider, for instance, the origin of the appendages in those insects which in the larval state possess neither legs nor wings, but exhibit a gradual emergence of these structures from concealment underneath the integumentary skeleton. In these cases, as I have already shown in regard to the wings, the development of the limbs arises from definite groups of cells in the skin. These must therefore be regarded as the formative, and therefore as the most important and indispensable, parts of the rudiments, and may be designated the imaginal disks, as I many years ago proposed[22] (Fig. 89, _ui_ and _oi_).

[22] _Die Entwicklung der Dipteren_, Leipzig, 1864.

But these disks of cells do not contain the _whole_ leg, but only the skin-layer of it, the 'hypodermis,' which, however, in this case undoubtedly determines the form. But the internal parts of the leg, especially the nerves, tracheæ, and probably also the muscles, are formed from other cell-groups and grow into the imaginal disk from outside. Something similar probably takes place in the case of all organs which are made up of many parts; they are, so to speak, shot together from several points of origin, from various primordia; and determinants are brought into co-operation whose relative value in determining the form and function of the organ may be very diverse.

For it is undoubtedly a very different matter whether a cell bears within it the elements which compel it in the course of growth to develop an organ, for instance a leg, of quite definite size, sculpture, number of joints, and so on, or whether it only bears the somewhat vague power of determining that connective tissue or fatty tissue is to be produced. In the first case it controls the whole formation of the part, in the second it only fills up gaps or lays down fat or other substances within itself if these be presented to it. Between these two extremes of determining power there are many intermediate stages. Cells which contain the determinants of blood-vessels, tracheæ, or nerves need not be so definitely determined that they always give rise to precisely the same blood-vessels, the same branching of the tracheæ, or the same bifurcation of nerves; they may probably possess no more than the general tendency to the formation of such parts, and the special form taken by the nerves, tracheæ, or blood-vessels may be essentially determined by their environment. Thus in the morbid tumours of Man, nerves, and especially blood-vessels, may develop in a quite characteristic manner, which was certainly not determined in advance, but has been called forth by the stimulus, the pressure, and other influences of the cellular basis of the tumour. In short, the cells were only determined to this extent, that they contained the tendency to give rise to blood-vessels under particular influences.

It would be a mistake, however, to think of the primary constituents of all cell-groups as so indefinite. Let us call to mind, for instance, the venation of the insect wing. It is well known that this is not only quite different in beetles, bugs, and Diptera from that in the Hymenoptera, and different again in the butterflies, but that it is quite characteristic in every individual family of butterflies, and indeed in every genus. We cannot conceive of the absolute certainty of development of these very characteristic and constant branchings as having its roots elsewhere than in the determinants of the germ-plasm, which, lying within certain series of cells, ultimately cause particular cell-series of the wing-rudiment to become the wing-veins. If this were not so, how would it be possible to understand the fact that every minute deviation in the course of these veins is repeated in exactly the same way in all the individuals of a genus, while in all the individuals of an allied genus the venation turns out slightly different with equal constancy.

But it is quite certain that all determinations are in some degree susceptible to modifying influences, that they are in very different degrees capable of variation.

Many deformities of particular parts in Man and the higher animals may be referred to imperfect or inhibited nutrition of the part in question during embryonic development; the determinants alone cannot make the part, they must have a supply of formative material, and according as this material is afforded more abundantly or more scantily the part will turn out larger or smaller. In the same way the pressure conditions of the surrounding parts must in many cases have a furthering or inhibiting influence, or may even determine the shape. But it is quite possible, indeed even probable, that other specific influences are exerted by the cells or cell-aggregates surrounding an organ which is in process of being formed, just as the stake on which a twining plant is growing may prompt it to coil. If the stake be absent, the predetermined twining of the plant cannot attain to more than very imperfect expression, if indeed it finds any. The spirally coiled sheath of muscle-cells which occurs so often around blood-vessels in worms, Echinoderms, and Vertebrates is probably due to similar processes, that is, on the one hand, to a specific mode of reaction characteristic of these cells, and predetermined from the germ; on the other hand, to the external influence of the cell-surroundings without which the determination of the muscle-cell is not liberated, that is, is not excited to activity.

But even if every determinant requires a stimulus to liberate it, whether this stimulus consists in currents of particular nutritive fluids, in contact with other cells, or, conversely, on the removal of some pressure previously exerted on the cell by its surroundings, the material cause of a structure is to be sought for not in these conditions of its appearance, but in the primary constituents which have been handed on to the relevant cell or cell-group from the germ, in other words, through its determinants. How, for instance, could the blunt rounded knob of the rough and clumsily jointed sac of cells which represents the insect's leg at the beginning of the pupal period (Fig. 93, _A_) be incited to thicken, to constrict at the root (_B_), and to form a joint-surface, to broaden out at the end, and produce two sharply cut points (_C_), which become incurved and form claws (_kr_), while beneath these a broad flat lobe (_hl_) grows forward, and with its regularly disposed cells gradually forms the characteristic adhesive organ of the fly--how could all this happen if there were not contained within these cells special formative forces which determine them not only in their form and the rest of their constitution, but above all in their power of multiplication? No special external stimulus affects the still unfinished knob of the fly's leg unless it be the removal of pressure; but this operates regularly, and cannot be the cause of the growth, at definite places, of claws and adhesive lobes with all their characteristically placed hairs.

We require to assume that each of the cells composing the primary rudiment of the limb possessed a determining power which made it grow and multiply under the given conditions of nutrition and pressure in a prescribed manner and at a prescribed rate; and we must make the same assumption in regard to all the daughter and grand-daughter-cells, and so on. The strictest regulation of the power of multiplication of each of the implicated cells is a necessary condition of the constant production of the same two claws and adhesive lobes, the same form of tarsal joint, the same regular covering of hair, and so on. This exact determination of the cells can only take place through material vital particles, and it is these which I call determinants.

I have already said so much about the assumed 'determinants' of the germ-plasm that it might perhaps be supposed that we have now exhausted the topic; but the assumption of such 'primary constituents' is so fundamental, not only for my own germ-plasm theory of to-day and to-morrow, but also--unless I am much mistaken--for all future theories of development and inheritance. In point of fact, the conception of determinants has as yet penetrated so little into the consciousness of biologists, that I cannot remain content with what I have already said, but must endeavour to test and to corroborate my thesis by additional illustrations.

As far as I am aware, only a few zoologists have expressly and unconditionally agreed with the assumption of determinants; on the other hand, several biologists have rejected it as fanciful and untenable, while others have set it aside as a useless playing with ideas. The last, I am inclined to believe, have not taken the trouble to think out what the idea is. It has even been objected that there can be no determinants because we can see nothing of them, and that they must therefore be pure figments of the imagination, invented to explain facts which could be explained much more easily and simply in some other way. From the very first I have stated emphatically that they have not been, and never will be seen, because they lie far below the limit of visibility, and thus can at best only become visible when they are collected in large aggregates like chromatin granules. Nor have I any objections to make if any one chooses to describe all the details of their activity as mere hypotheses, such, for instance, as their distribution during development, their 'maturation,' their migration from the nucleus, and the manner in which they control the cell. All this is really an imaginative picture which may be correct to a certain degree, but may also be erroneous; no formal proof of it can be obtained at present; and I am content if it be simply admitted to be possible. On the other hand, the existence of determinants seems to me to be, in the sense indicated, indubitable and demonstrable.

Let us return for a moment to the claws and adhesive lobes which are developed on the foot of the fly. It may perhaps be thought that it is possible to do without the assumption of determinants for these parts, by assuming that although 'external' influences in the ordinary sense could not possibly have determined that certain cells of the apex of the leg should form claws and others adhesive lobes, the result might be due to the differences of intercellular pressure within the apical knob; these may have been stronger in one direction, weaker in another, thus prompting the cells to grow here into claws and there into adhesive lobes. If we had merely to explain from the constitution of the germ-plasm the ontogeny or development of these parts in an individual fly there might perhaps be no radical objection to this view, though it would hardly be possible to explain the assumed differences in pressure otherwise than as due to a different intensity of growth in the cells in the various regions of the limb-apex, which again would have to be referred to differences in the germ-plasm. But when we reflect that these parts vary hereditarily and independently of other parts, and owe their present form to their power of doing so, and that they are differently formed in every genus and species, we see at once that they must be represented in the germ-plasm by particular vital particles, which are the roots of their transmissible variability, that is, which must have previously undergone a corresponding variation if the relevant parts themselves are to vary. Without previous variation of the determinants of the germ no transmissible independent deviation on the part of the claws or adhesive lobes of the animal is conceivable.

All the opponents of my theory have overlooked this fact; both Oscar Hertwig and Kassowitz have forgotten that a theory of development is not a theory of heredity; they only aim at the former, and they therefore dispute the logical necessity for an assumption of determinants.

But as this is the very foundation of the theory, let me further submit the following considerations in its favour.

In insects which undergo metamorphosis, not only the external but the internal parts of the caterpillar or larva go through a more or less complete transformation. In the flies (Muscidæ), for instance, the whole intestinal tract of the larva is reconstructed in the pupa; in fact it breaks up into a loose, flocculent, dead, but still coherent mass of tissue. Within this there arises a new intestine, as I have shown in an early work (1864); and Kowalewsky and Van Rees have since made us aware of the interesting details of this reconstruction, showing that the new intestine arises from definite cells of the old one, which are present in the larval gut at certain fairly wide distances, and which do not share in the general destruction, but remain alive, grow, and multiply, and form islands of cells in the dead mass. These living islands, continually extending, ultimately come into contact and again form a closed intestinal canal which differs entirely from that of the larva in its form, in its various areas, and in its differentiation. In this case those formative cells of the imago-intestine must have contained the elements which determined their descendants in number, power of multiplication, arrangement, and histological differentiation. In other words, each of these cells must contain the determinants of a particular limited section of the intestine of the imago. The other cells of the intestinal epithelium could not do this, even though they were under exactly the same conditions, were included in the same intimate cell-aggregate, and had the same nutritional opportunities. They break up when the formative cells begin to be active, for till then the latter had remained inactive, and had not multiplied, although they lay regularly distributed among the other cells. Whence, then, could the entire difference in the behaviour of these two sets of cells arise, if it does not depend on the _nature of the cells themselves_, and how could this difference of nature have developed during the racial history of insect-metamorphosis if determinants did not reach the cell from the germ-plasm--determinants which conditioned that some cells should be hereditarily modified into the cells of the imago-intestine and others into the larval intestine? Quite similar processes have been recently demonstrated in regard to the reconstruction of the larval intestine in other insect-groups. Deegener has done this, for instance, for the water-beetle (_Hydrophilus piceus_); and it is certain that all these reconstructions start from particular cells, which lie indifferently between the active cells during the larval period, and contain the primary constituents for the formation of a section of the intestine, but which only become active when their hitherto living neighbours die and break up.