CHAPTER V.

_ON THE ORIGIN OF INSECTS._

"Personne," says Carl Vogt, "en Europe au moins, n'ose plus soutenir la Création indépendante et de toutes pièces des espèces," and though this statement is perhaps not strictly correct, still it is no doubt true, that the Doctrine of Evolution, in some form or other, is accepted by most, if not by all, the greatest naturalists of Europe. Yet it is surprising how much, in spite of all that has been written, Mr. Darwin's views are still misunderstood. Thus Browning, in one of his recent poems, says:--

"That mass man sprang from was a jelly lump Once on a time; he kept an after course Through fish and insect, reptile, bird, and beast, Till he attained to be an ape at last, Or last but one."[53]

This theory, though it would be regarded by many as a fair statement of his views, is one which Mr. Darwin would entirely repudiate. Whether fish and insect, reptile, bird and beast, are derived from one original stock or not, they are certainly not links in one sequence. I do not, however, propose to discuss the question of Natural Selection, but may observe that it is one thing to acknowledge that in Natural Selection, or the survival of the fittest, Mr. Darwin has called attention to a _vera causa_, has pointed out the true explanation of certain phenomena; but it is quite another thing to maintain that all animals are descended from some primordial source.

For my own part, I am satisfied that Natural Selection is a true cause, and, whatever may be the final result of our present inquiries--whether animated nature be derived from one ancestral source, or from many--the publication of the Origin of Species will none the less have constituted an epoch in the History of Biology. But, how far the present condition of living beings is due to that cause; how far, on the other hand, the action of Natural Selection has been modified and checked by other natural laws--by the unalterability of types, by atavism, &c.; how many types of life originally came into being; and whether they arose simultaneously or successively,--these and many other similar questions remain unsolved, even admitting the theory of Natural Selection. All this has indeed been clearly pointed out by Mr. Darwin himself, and would not need repetition but for the careless criticism by which in too many cases the true question has been obscured. Without, however, discussing the argument for and against Mr. Darwin's conclusions, so often do we meet with travesties of it like that which I have just quoted, that it is well worth while to consider the stages through which some group, say for instance that of insects, have probably come to be what they are, assuming them to have developed under natural laws from simpler organisms. The question is one of great difficulty. It is hardly necessary to say that insects cannot have passed through all the lower forms of animal life, and naturalists do not at present agree as to the actual line of their development.

In the case of insects, the gradual course of evolution through which the present condition of the group has probably been reached, has been discussed by Mr. Darwin, by Fritz Müller, Haeckel, Brauer, myself and others.

In other instances Palæontology throws much light on this question. Leidy has shown that the milk-teeth of the genus _Equus_ resemble the permanent teeth of the ancient _Anchitherium_, while the milk-teeth of _Anchitherium_ again approximate to the dental system of the still earlier _Merychippus_. Rütimeyer, while calling attention to this interesting observation, adds that the milk-teeth of _Equus caballus_ in the same way, and still more those of _E. fossilis_, resemble the permanent teeth of _Hipparion_.

"If we were not acquainted with the horse," says Flower,[54] "we could scarcely conceive of an animal whose only support was the tip of a single toe on each extremity, to say nothing of the singular conformation of its teeth and other organs. So striking have these characters appeared to many zoologists, that the animals possessing them have been reckoned as an order apart, called Solidungula; but palæontology has revealed that in the structure of its skull, its teeth, its limbs, the horse is nothing more than a modified _Palæotherium_; and though still with gaps in certain places, many of the intermediate stages of these modifications are already known to us, being the _Palæotherium_, _Anchitherium_, _Merychippus_, and _Hipparion_."

"All Echinoids," says A. Agassiz,[55] "pass, in their early stages, through a condition which recalls to us the first Echinoids which made their appearance in geological ages." On embryological grounds, he observes, we should "place true Echini lowest, then the Clypeastroids, next the Echinolamps, and finally the Spatangoids." Now among the Echinoids of the Trias there are no Clypeastroids, Echinolamps, or Spatangoids. The Clypeastroids make their appearance in the Lias, the Echinolamps in the Jurassic, while the Spatangoids commence in the Cretaceous period.

Again[56] "in the Radiates, the Acalephs in their first stages of growth, that is, in their Hydroid condition, remind us of the adult forms among Polyps, showing the structural rank of the Acalephs to be the highest, since they pass beyond a stage which is permanent with the Polyps; while the Adult forms of the Acalephs have in their turn a certain resemblance to the embryonic phases of the class next above them, the Echinoderms; within the limits of the classes, the same correspondence exists as between the different orders; the embryonic forms of the highest Polyps recall the adult forms of the lower ones, and the same is true of the Acalephs as far as these phenomena have been followed and compared among them." Indeed, the accomplished authors from whom I have taken the above quotation, do not hesitate to say[57] that "whenever such comparisons have been successfully carried out, the result is always the same; the present representatives of the fossil types recall in their embryonic condition the ancient forms, and often explain their true position in the animal kingdom."

Fossil insects are unfortunately rare, there being but few strata in which the remains of this group are well preserved. Moreover, well-characterized Orthoptera and Neuroptera occur as early as the Devonian strata; Coleoptera and Hemiptera in the Coal-measures; Hymenoptera and Diptera in the Jurassic; Lepidoptera, on the contrary, not until the Tertiary. But although it appears from these facts that, as far as our present information goes, the Orthoptera and Neuroptera are the most ancient orders, it is not, I think, conceivable that the latter should have been derived from any known species of the former; on the other hand, the earliest known Neuroptera and Orthoptera, though in some respects less specialized than existing forms, are as truly, and as well characterized, Insects, as any now existing; nor are we acquainted with any earlier forms, which in any way tend to bridge over the gap between them and lower groups, though, as we shall see, there are types yet existing which throw much light on the subject.

In the consideration then of this question, we must rely principally on Embryology and Development. I have already referred to the cases in which species, very unlike in their mature condition, are very similar one to another when young. Haeckel, in his "Naturliche Schöpfungsgeschichte," gives a diagram which illustrates this very well as regards Crustacea. Pls. 1-4 show the same to be the case with Insects.

The Stag-beetle, the Dragon-fly, the Moth, the Bee, the Ant, the Gnat, the Grasshopper,--these and other less familiar types seem at first to have little in common. They differ in size, in form, in colour, in habits, and modes of life. Yet the researches of entomologists, following the clue supplied by the illustrious Savigny, have proved, not only that while differing greatly in details, they are constructed on one common plan; but also that other groups, as for instance, Crustacea (Lobsters, Crabs, &c.) and Arachnida (Spiders and Mites), can be shown to be fundamentally similar. In Pl. 4 I have figured the larvæ of an _Ephemera_ (Fig. 1), of a _Meloë_ (Fig. 2), of a Dragon-fly (Fig. 3), of a Sitaris (Fig. 4), of a _Campodea_ (Fig. 5), of a _Dyticus_ (Fig. 6), of a Termite (Fig. 7), of a _Stylops_ (Fig. 8), and of a _Thrips_ (Fig. 9). All these larvæ possess many characters in common. The mature forms are represented in the corresponding figures of Plate 3, and it will at once be seen how considerably they differ from one another. The same fact is also illustrated in Figs. 48-55, where Figs. 48-51 represent the larval states of the mature forms represented in Figs. 52-55. Fig. 48 is the larva of a moth, _Agrotis suffusa_ (Fig. 52); Fig. 49 of a beetle, _Haltica_ (Fig. 53); Fig. 50 of a Saw-fly, _Cimbex_ (Fig. 54); and Fig. 51 of a Centipede, _Julus_ (Fig. 55).

Thus, then, although it can be demonstrated that perfect insects, however much they differ in appearance, are yet reducible to one type, the fact becomes much more evident if we compare the larvæ. M. Brauer[58] and I[59] have pointed out that two types of larvæ, which I have proposed to call _Campodea_-form and _Lindia_-form, and which Packard has named Leptiform and Eruciform, run through the principal groups of insects. This is obviously a fact of great importance: as all individual _Meloës_ are derived from a form resembling Pl. 2, Fig. 2, it is surely no rash hypothesis to suggest that the genus itself may have been so.

Firstly, however, let me say a word as to the general Insect type. It may be described shortly as consisting of animals possessing a head, with mouth parts, eyes and antennæ; a many segmented body, with three pairs of legs on the segments immediately following the head; with, when mature, either one or two pairs of wings, generally with caudal appendages I will not now enter into a description of their internal anatomy. It will be seen that, except as regards the wings, Pl. 4, Fig. 4, representing the larva of a small beetle named _Sitaris_, answers very well to this description. Many other Beetles are developed from larvæ closely resembling those of _Meloë_ (Pl. 4, Fig. 2), and Sitaris (Pl. 4, Fig. 4); in fact--except those species the larvæ of which, as, for instance of the Weevils (Pl. 2, Fig. 6), are internal feeders, and do not require legs--we may say that the Coleoptera generally are derived from larvæ of this type.

I will now pass to a second order, the Neuroptera. Pl. 4, Fig. 1, represents the larva of _Chloëon_, a species the metamorphoses of which I described some years ago in the Linnean Transactions,[60] and it is obvious that in essential points it closely resembles the form to which I have just alluded.

The Orthoptera, again, the order to which Grasshoppers, Crickets, Locusts, &c. belong, commence life in a similar condition; and the same may also be said of the Trichoptera.

The larvæ of Bees when they quit the egg are entirely legless, but in an earlier stage they possess well-marked rudiments of thoracic legs, showing, as it seems to me, that their apodal condition is an adaptation to their circumstances. Other Hymenopterous larvæ, those for example of _Sirex_ (Fig. 9), and of the Saw-flies (Fig. 50) have well-developed thoracic legs.

From the difference in external form, and especially from the large comparative size of the abdomen, these larvæ, as well as those of Lepidoptera (Fig. 48), have generally been classed with the maggots of Flies, Weevils, &c., rather than with the more active form of larva just adverted to. This seems to me, as I have already pointed out,[61] to be a mistake. The caterpillar type differs, no doubt, in its general appearance, owing to its greater clumsiness, but still essentially agrees with that already described.

No Dipterous larva, so far as I know, belongs truly to this type; in fact, the early stages of the pupa in the Diptera seem in some respects to correspond to the larvæ of other Insect orders. The Development of the Diptera is, however, as Weissman[62] has shown, very abnormal in other respects.

Thus, then, we find in many of the principal groups of insects that, greatly as they differ from one another in their mature condition, when they leave the egg they more nearly resemble the typical insect type; consisting of a head; a three-segmented thorax, with three pairs of legs; and a many-jointed abdomen, often with anal appendages. Now, is there any mature animal which answers to this description? We need not have been surprised if this type, through which it would appear that insects must have passed so many ages since (for winged Neuroptera have been found in the carboniferous strata) had long ago become extinct. Yet it is not so. The interesting genus _Campodea_ (Pl. 3, Fig. 5) still lives; it inhabits damp earth, and closely resembles the larva of _Chloëon_ (Pl. 2, Fig. 1), constituting, indeed, a type which, as shown in Pl. 4, occurs in many orders of insects. It is true that the mouth-parts of _Campodea_ do not resemble either the strongly mandibulate form which prevails among the larvæ of Coleoptera, Orthoptera, Neuroptera, Hymenoptera, Lepidoptera; or the suctorial type of the Homoptera and Heteroptera. It is, however, not the less interesting or significant on that account, since, as I have elsewhere[63] pointed out, its mouth-parts are intermediate between the mandibulate and haustellate types; a fact which seems to me most suggestive.

It appears, then, that there are good grounds for considering that the various types of insects are descended from ancestors more or less resembling the genus _Campodea_, with a body divided into head, thorax, and abdomen: the head provided with mouth-parts, eyes, and one pair of antennæ; the thorax with three pairs of legs; and the abdomen, in all probability, with caudal appendages.

If these views are correct, the genus _Campodea_ must be regarded as a form of remarkable interest since it is the living representative of a primæval type, from which not only the Collembola and Thysanura, but the other great orders of insects have derived their origin.

From what lower group the _Campodea_ type was itself derived is a question of great difficulty. Fritz Müller indeed says,[64] "if all the classes of Arthropoda (Crustacea, Insecta, Myriopoda, and Arachnida) are indeed all branches of a common stem (and of this there can scarcely be a doubt), it is evident that the water-inhabiting and water-breathing Crustacea must be regarded as the original stem from which the other terrestrial classes, with their tracheal respiration, have branched off." Haeckel, moreover, is of the opinion that the Tracheata are developed from the Crustacea, and probably from the Zoëpoda. For my own part, though I feel very great diffidence in expressing an opinion at variance with that of such high authorities, I am rather disposed to suggest that the _Campodea_ type may possibly have been derived from a less highly developed one, resembling the modern Tardigrade,[65] a (Fig. 56) smaller and much less highly organized being than _Campodea_. It possesses two eyes, three anterior pairs of legs, and one at the posterior end of the body, giving it a curious resemblance to some Lepidopterous larvæ.

These legs, however, as will be seen, are reduced to mere projections. But for them, the Tardigrada would closely resemble the vermiform larva so common among insects. Among Trichoptera the larva early acquires three pairs of legs, but as Zaddach has shown,[66] there is a stage, though it is quickly passed through, in which the divisions of the body are indicated, but no trace of legs is yet present. Indeed, there appear to be reasons for considering that while among Crustacea the appendages appear before the segments, in Insects the segments precede the appendages, although this stage of development is very transitory, and apparently, in some cases, altogether suppressed. I say "apparently," because, as I have already mentioned, I am not yet satisfied that it will not eventually be found to be so in all cases. Zaddach, in his careful observations of the embryology of _Phryganea_, only once found a specimen in this stage, which also, according to the researches of Huxley,[67] seems to be little more than indicated in _Aphis_. It is therefore possible that in other cases, when no such stage has been observed, it not really may be absent, but, from its transitoriness, may have hitherto escaped attention.

Fritz Müller has expressed the opinion[68] that this vermiform type is of comparatively recent origin. He says: "The ancient insects approached more nearly to the existing Orthoptera, and perhaps to the wingless Blattidæ, than to any other order, and the complete metamorphosis of the Beetles, Lepidoptera, &c., is of later origin." "There were," he adds, "perfect insects before larvæ and pupæ." This opinion has been adopted by Mr. Packard[69] in his "Embryological Studies on Hexapodous Insects."

M. Brauer[70] also considers that the vermiform larva is a more recent type than the Hexapod form, and is to be regarded not as a developmental form, but as an adaptational modification of the earlier active hexapod type. In proof of this he quotes the case of _Sitaris_.

Considering, however, the peculiar habits of this genus, to which I have already referred, and also that the vermiform type is altogether lower in organization and less differentiated than the _Campodea_ form, I cannot but regard this case as exceptional; one in which the development has been, as it were, to use an expression of Fritz Müller's, "falsified" by the struggle for existence, and which therefore does not truly indicate the successive stages of evolution. On the whole, the facts seem to me to point to the conclusion that, though the grub-like larvæ of Coleoptera and some other insects, owe their present form mainly to the influence of external circumstances, and partially also to atavism, still the _Campodea_ type is itself derived from earlier vermiform ancestors. Nicolas Wagner has shown in the case of a small gnat, allied to _Cecidomyia_, that even now, in some instances, the vermiform larvæ possess the power of reproduction. Such a larva (as, for instance, Fig. 57) very closely resembles some of the Rotatoria, such for instance as _Albertia_ or _Notommata_, which however possess vibratile cilia. There is, indeed, one genus--_Lindia_ (Fig. 58)--in which these ciliæ are altogether absent, and which, though resembling _Macrobiotus_ in many respects, differs from that genus in being entirely destitute of legs. I have never met with it myself, but it is described by Dujardin, who found it in a ditch near Paris, as being oblong, vermiform, divided into rings, and terminating posteriorly in two short conical appendages. The jaws are not unlike those of the larvæ of Flies, and indeed many naturalists meeting with such a creature would, I am sure, regard it as a small Dipterous larva; yet Dujardin figures a specimen containing an egg, and seems to have no doubt that it is a mature form.[71]

For the next descending stage we must, I think, look among the Infusoria, through such genera as _Chætonotus_ or _Ichthydium_. Other forms of the Rotatoria, such for instance as _Rattulus_, and still more the very remarkable species discovered in 1871 by Mr. Hudson,[72] and described under the name of _Pedalion mira_, seem to lead to the Crustacea through the Nauplius form. Dr. Cobbold tells me that he regards the _Gordii_ as the lowest of the Scolecida; Mr. E. Ray Lankester considers some of the Turbellaria, such genera as _Mesostomum_, _Vortex_, &c., to be the lowest of existing worms; excluding the parasitic groups. Haeckel[73] also regards the Turbellaria as forming the nearest approach to the Infusoria. The true worms seem, however, to constitute a separate branch of the animal kingdom.

We may take, as an illustration of the lower worms, the genus Prorhynchus (Fig. 59), which consists of a hollow cylindrical body, containing a straight simple tube, the digestive organ.

But however simple such a creature as this may be, there are others which are far less complex, far less differentiated; which therefore, on Mr. Darwin's principles, may be considered still more closely to represent the primæval ancestor from which these more highly-developed types have been derived, and which, in spite of their great antiquity--in spite of, or perhaps in consequence of, their simplicity, still maintain themselves almost unaltered.

Thus the form which Haeckel has described[74] under the name _Protamoeba primitiva_, Pl. 5, Fig. 1-5, consists of a homogeneous and structureless substance, which continually alters its form; putting out and drawing in again more or less elongated processes, and creeping about like a true _Amoeba_, from which, however, _Protamoeba_ differs, in the absence of a nucleus. It seems difficult to imagine anything simpler; indeed, as described, it appears to be an illustration of properties without structure. It takes into itself any suitable particle with which it comes in contact, absorbs that which is nutritious, and rejects the rest. From time to time a constriction appears at the centre (Pl. 5, Fig. 2), its form approximates more and more to that of an hour-glass (Pl. 5, Fig. 3), and at length the two halves separate, and each commences an independent existence (Pl. 5, Fig. 5).

In the true _Amoebas_, on the contrary, we find a differentiation between the exterior and the interior: the body being more or less distinctly divisible into an outer layer and an inner parenchyme. In the _Amoebas_, as in _Protamoeba_, multiplication takes place by self-division, and nothing corresponding to sexual reproduction has yet been discovered.

Somewhat more advanced, but still of great simplicity, is the _Protomyxa aurantiaca_ (Pl. 5, Fig. 8), discovered by Haeckel[76] on dead shells of _Spirula_, where it appears as a minute orange speck, which shows well against the clear white of the _Spirula_. Examined with a microscope, the speck is seen to be a spherical mass of orange-coloured, homogeneous, albuminous matter, surrounded by a delicate, structureless membrane. It is obvious from this description that these bodies closely resemble eggs, for which indeed Haeckel at first mistook them. Gradually, however, the yellow sphere broke itself up into smaller spherules (Pl. 5, Fig. 9), after which the containing membrane burst, and the separate spherules, losing their globular form, crept out as small _Amoebæ_ (Pl. 5, Fig. 6), or amoeboid bodies. These little bodies moved about, assimilated the minute particles of organic matter, with which they came in contact, and gradually increased in size (Pl. 5, Fig. 7) with more or less rapidity according to the amount of nourishment they were able to obtain. They threw out arms in various directions, and if divided each section maintained its individual existence. After a while their movements ceased, they contracted into a ball, and again secreted round themselves a clear structureless envelope.

This completes their life history as observed by Haeckel, who found it easy to retain them in his glasses in perfect health, and who watched them closely.

As another illustration I may take the _Magosphæra planula_, discovered by Haeckel on the coast of Norway.

In one stage of its existence (Pl. 5, Fig. 10) it is a minute mass of gelatinous matter, which continually alters its form, moves about, feeds, and in fact behaves altogether like the _Amoeba_ just described. It does not, however, remain always in this condition. After a while it contracts into a spherical form (Pl. 5, Fig. ii), and secretes round itself a structureless envelope, which, with the nucleus, gives it a very close resemblance to a minute egg.

Gradually the nucleus divides, and the protoplasm also separates into two spherules (Pl. 5, Fig. 12); these two subdivide into four (Pl. 5, Fig. 13), and so on (Pl. 5, Fig 14), until at length thirty-two are present, compressed into a more or less polygonal form (Pl. 5, Fig. 15). Here this process ends. The separate spherules now begin to lose their smooth outline, to throw out processes, and to show amoeboid movements like those of the creatures just described. The processes or pseudopods grow gradually longer, thinner, and more pointed. Their movements become more active, until at length they take the form of ciliæ. The spherical _Magosphæra_, the upper surface of which has thus become covered with ciliæ, now begins to rotate within the cyst or envelope, which at length gives way and sets free the contained sphere, which then swims about freely in the water (Pl. 5, Fig. 16), thus closely resembling _Synura_, or one of the Volvocineæ. After swimming about in this condition for a certain time, the sphere breaks up into the separate cells of which it is composed (Pl. 5, Fig. 17). As long as the individual cells remained together, they had undergone no changes of form, but after separating they show considerable contractility, and gradually alter their form, until they become undistinguishable from true _Amoebæ_ (Pl. 5, Fig. 18). Finally, according to Haeckel, these amoeboid bodies, after living for a certain time in this condition, return to a state of rest, again contract into a spherical form, and secrete round themselves a structureless envelope. The life history of some other low organisms, as for instance _Gregarina_, is of a similar character.

It may be said, and said truly, that the difference between such beings as these and the _Campodea_, or Tardigrade, is immense. But if it be considered incredible that even during the long lapse of geological time such great changes should have taken place as are implied in the belief that there is genetic connection between them and these lower groups, let us consider what happens under our eyes in the development of each one of these little creatures in the proverbially short space of their individual life.

I will take for instance the first stages, and for the sake of brevity only the first stages, of the life-history of a Tardigrade.[77] As shown in Fig. 60, the egg is at first a round body or cell, with a clear central nucleus--the germinal vesicle; it increases in size, and after a while the yolk and the germinal vesicle divide into two (Fig. 61), then into four (Fig. 62), and so on, just as we have seen to be the case in _Magosphæra_. From the minute cells (Fig. 63) arising through this process of yolk-segmentation, the body of the Tardigrade is then built up.[78]

Though I will not now attempt to point out the full bearing of these facts on the study of embryology generally, yet I cannot resist calling attention to the similarity of the development of _Magosphæra_ with the first stages of development of other animals, because it appears to me to possess a significance, the importance of which it would be difficult to overestimate.

Among the Zoophytes Prof. Allman thus describes[79] the process in _Laomedea_, as representing the Hydroids (Pl. 6, Fig. 1, represents the young egg):--"The first step observable in the segmentation-process is the cleavage of the yolk into two segments (Pl. 6, Fig. 2), immediately followed by the cleavage of these into other two, so that the vitellus is now composed of four cleavage spheres (Pl. 6, Fig. 3)." These spheres again divide (Pl. 6, Fig. 4) and subdivide, thus at length forming minute cells, of which the body of the embryo is built up.

In Pl. 6, Figs. 5-9 represent the corresponding stages in the development of a small parasitic worm--the _Filaria mustelarum_--as given by Van Beneden.[80] The first process is that within the egg, which represents, so to say, the encysted condition of _Magosphæra_, the yolk divides itself into two balls (Pl. 6, Fig. 6), then into four, eight, and so on, the cells thus constituted finally forming the young worm. I have myself observed the same stages in the eggs of the very remarkable and abnormal _Sphærularia bombi_.[81]

Among the Echinoderms M. Derbès thus describes the first stages (Pl. 6, Figs. 10-13) in the development of the egg of an _Echinus_ (_Echinus esculentus_):--"Le jaune commence à se segmenter, d'abord en deux, puis en quatre et ainsi de suite, chacune des nouvelles cellules se partageant à son tour en deux."[82] Sars has observed the same thing in the starfish.[83]

In the Rotatoria, as shown by Huxley in _Lacinularia_,[84] and by Williamson in _Melicerta_,[85] the yolk is at first a single globular mass, the first changes which take place in it being as follows:--"The central nucleus becomes drawn out and subdivides into two, this division being followed by a corresponding segmentation of the yolk. The same process is repeated again and again, until at length the entire yolk is converted into a mass of minute cells." Among the Crustacea the total segmentation of the yolk occurs among the Copepoda, Rhizocephala, and Cirripedia. Sars has described the same process in one of the nudibranchiate mollusca[86] (_Tritonia_), Müller in Entochocha,[87] Haeckel in Ascidia,[88] Lacaze Duthiers in _Dentalium_.[89] Figures 18 to 21, Pl. 6, are taken from Koren and Danielssen's[90] memoir on the development of _Purpura lapillus_.

Figs. 22-24 show the same stages in a fish (_Amphioxus_) as given by Haeckel, and it is unnecessary to point out the great similarity.

Lastly, figures 25 to 29, Pl. 6, are given by Dr. Allen Thomson,[91] as illustrating the first stages in the development of the vertebrata.

I might have given many other examples, but the above are probably sufficient, and will show that the processes which constitute the life-history of the lowest organized beings very closely resemble the first stages in the development of more advanced groups; that as Allen Thomson has truly observed,[92] "the occurrence of segmentation and the regularity of its phenomena are so constant that we may regard it as one of the best established series of facts in organic nature."

It is true that normal yolk-segmentation is not universal in the animal kingdom; that there are great groups in which the yolk does not divide in this manner,--perhaps owing to some difference in its relation to the germinal vesicle, or perhaps because one of the suppressed stages in embryological development, many examples might be given, not only in zoology, but, as I may state on the authority of Dr. Hooker, in botany also. But, however, this may be, it is surely not uninteresting, nor without significance, to find that changes which constitute the life-history of the lowest creatures for the initial stages even of the highest.

Returning, in conclusion, to the immediate subject of this work, I have pointed out that many beetles and other insects are derived from larvæ closely resembling _Campodea_.

Since, then, individual insects are certainly in many cases developed from larvæ closely resembling the genus _Campodea_, why should it be regarded as incredible that insects as a group have gone through similar stages? That the ancestors of beetles under the influence of varying external conditions, and in the lapse of geological ages, should have undergone changes which the individual beetle passes through under our own eyes and in the space of a few days, is surely no wild or extravagant hypothesis. Again, other insects come from vermiform larvæ much resembling the genus _Lindia_, and it has been also repeatedly shown that in many particulars the embryo of the more specialized forms resembles the full-grown representatives of lower types. I conclude, therefore, that the Insecta generally are descended from ancestors resembling the existing genus _Campodea_, and that these again have arisen from others belonging to a type represented more or less closely by the existing genus _Lindia_.

Of course it may be argued that these facts have not really the significance which they seem to me to possess. It may be said that when Divine power created insects, they were created with these remarkable developmental processes. By such arguments the conclusions of geologists were long disputed. When God made the rocks, it was tersely said, He made the fossils in them. No one, I suppose, would now be found to maintain such a theory; and I believe the time will come when it will be generally admitted that the structure of the embryo, and its developmental changes, indicate as truly the course of organic development in ancient times as the contents of rocks and their sequence teach us the past history of the earth itself.

FOOTNOTES:

[1] Darwin's "Researches into the Geology and Natural History of the Countries visited by H.M.S. _Beagle_," p. 326.

[2] Introduction to Entomology, vi. p. 50.

[3] Manual of Entomology, p. 30.

[4] Linnean Journal, vol. xi.

[5] Introduction to the Modern Classification of Insects, p. 17.

[6] Linnean Transactions, 1863--"On the Development of _Chloëon_."

[7] The figures on the first four plates are principally borrowed from Mr. Westwood's excellent "Introduction to the Modern Classification of Insects."

[8] "Sur la Domestication des _Clavigers_ par les Fourmis." Bull. de la Soc. d'Anthropologie de Paris, 1868, p. 315.

[9] Westwood's Introduction, vol. i. p. 36.

[10] Westwood's Introduction, vol. ii. p. 52.

[11] Die Fortpflanzung und Entwickelung der Pupiparen. Von Dr. R. Leuckart. Halle. 1848.

[12] Ann. des Sci. Nat., sér. 4, tome vii. See also _Natural History Review_, April 1862.

[13] Ann. and Mag. of Nat. Hist. 1852.

[14] Zeits. für Wiss. Zool. 1869.

[15] Transactions of the Linnean Society, 1863.

[16] Lectures on the Anatomy, &c. of the Invertebrate Animals.

[17] Untersuchungen über die Entwickelung und den Bau der Gliederthiere, 1854.

[18] Linnean Transactions, vol. xxii. 1858.

[19] "Embryological Studies on Hexapodous Insects." Peabody Academy of Science. Third Memoir.

[20] Mém. de l'Acad. Imp. des Sci. de St. Pétersbourg. 1869.

[21] Observationes de Prima Insectorum Genesi, p. 14.

[22] Mém. de l'Acad. Imp. des Sci. de St. Pétersbourg. tome xvi. 1871, p. 35.

[23] Recherches sur l'Evolution des Araignées.

[24] Philosophical Transactions, 1841.

[25] Monog. of the Gymnoblastic or Tubularian Hydroids. See also Hincks, British Hydroid Zoophytes. Pl. x.

[26] Loc. cit. p. 315.

[27] Philosophical Transactions, 1859, p. 589.

[28] "Facts for Darwin," Eng. Trans. p. 127.

[29] Rolleston, "Forms of Animal Life," p. 146.

[30] A. Agassiz, "Embryology of the Starfish," p. 25; "Embryology of Echinoderms." Mem. of Am. Ac. of Arts and Sciences N.S. vol. ix. p. 9.

[31] Ueber die Gattungen der Seeigellarven. Siebente Abhandlung. Kön. Akad. d. Wiss. zu Berlin. Von Joh. Müller, 1855, Pl. iii. fig. 3.

[32] Huxley, Introduction to the Classification of Animals, p. 45.

[33] Philosophical Transactions, 1865 and 1866.

[34] Loc. cit. Zweit. Abh. Pl. i., figs. 8 and 9.

[35] Thomson, on the Embryology of the Echinodermata, _Natural History Review_, 1863, p. 415. See also Agassiz, "Embryology of the Starfish," p. 62.

[36] A. Agassiz, Embryology of Echinoderms, p. 18.

[37] Hincks. British Hydroid Zoophytes, pp. 120-147.

[38] Zeits. für Wiss. Zool. 1864, p. 228.

[39] Introduction to Entomology, 6th ed. vol. i. p. 61.

[40] Métamorphoses de l'Homme et des Animaux, p. 133. See also Carpenter, Principles of Physiology. 1851, p. 389.

[41] Darwin, Origin of Species, 4th ed. p. 532.

[42] Principles of Biology, vi. p. 349.

[43] For differences in larva consequent on variation in the external condition, see _ante_, p. 61.

[44] See Hincks. British Hydroid Zoophytes, P. lxii. Agassiz, Sea-side Studies, p. 43.

[45] See Newport, Phil. Trans., 1832.

[46] Linnean Transactions, 1862.

[47] Origin of Species, 4th ed., pp. 14 and 97.

[48] On the Alternation of Generations. By J. J. Steenstrup. Trans. by C. Busk, Esq. Ray Society. 1842.

[49] Zeit. für Wiss. Zool. 1863.

[50] Mém. de l'Acad. Imp. de St. Pétersbourg. vol. xv. 1870.

[51] Of course all animals in which the sexes are distinct are in one sense dimorphic.

[52] "There is no such thing as a true case of 'alternation of generations in the animal kingdom;' there is only an alternation of true generation with the totally distinct process of gemmation or fission."--HUXLEY _on Animal Individuality_, Ann. and Mag. of Nat. Hist. June 1852.

[53] Prince Hohenstiel Schwangau, p. 68.

[54] Journal of the Royal Institution. April 1873.

[55] "Embryology of Echinoderms," l. c. p. 15.

[56] Mr. and Mrs. Agassiz: "Sea-side Studies," p. 139.

[57] l. c. p. 138.

[58] Wien. Zool. Bot. Gesells, 1869.

[59] Linnean Transactions, 1863.

[60] Linnean Transactions, 1866, vol. xxv.

[61] Linnean Transactions, vol. xxiv. p. 65.

[62] Siebold und Kolliker's Zeitschr. f. Wiss. Zool., 1864.

[63] Linnean Journal, vol. xi.

[64] Facts for Darwin, p. 120.

[65] A still nearer approach is afforded by the genus _Peripatus_, which since the above was written has been carefully described, especially by Moseley and Hutton. There are several species, scattered over the southern hemisphere. In general appearance they look like a link between a caterpillar and a centipede. They have a pair of antennæ, two pairs of jaws, and (according to the species) from fourteen to thirty-three pairs of legs. They breathe by means of tracheæ, which open diffusely all over the body.

[66] Unters. üb. die Entwick, und den Bau der Gliederthiere, p. 73.

[67] Linnean Transactions, v. xxii.

[68] Facts for Darwin, trans. by Dallas, p. 118. See also Darwin, "Origin of Species," p. 530. 4th ed.

[69] Mem. Peabody Academy of Science, v. I. No, 3.

[70] Wien. Zool. Bott. Gesells. 1869, p. 310.

[71] See also the descriptions given by Dujardin (Ann. des Sci. Nat. 1851, v. xv.) and Claparède (Anat. und Entwickl. der Wirbel osen Thiere) of the interesting genus _Echinoderes_, which these two eminent naturalists unite in regarding as intermediate between the Annelides and the Crustacea.

[72] "On a New Rotifer." _Monthly Microscopical Journal_, Sept. 1871.

[73] Generelle Morphologie, vol. ii. p. 79.

[74] Monographie der Moneren, p. 43.

[75] Gegenbaur. Grund. d. Vergleich. Anat. p. 210. See also Dr. M. S. Schultze, Beiträge zur Naturg. der. Turbellarien. 1851. Pl. vi. fig. 1.

[76] Monographieder Moneren, p. 10.

[77] See Kauffmann, Ueber die Entwickelung and systematische Stellung der Tardigraden. Zeits. f. Wiss. Zool. 1851, p. 220.

[78] It is true that among the Insecta generally the first stages of development differ in appearance considerably from those above described; those of _Platygaster_, as figured by Ganin (ante Figs. 17-22), being very exceptional.

[79] Monograph of the Gymnoblastic or Tubularian Hydroids, by G. J. Allman, Ray Soc. 1871, p. 86.

[80] Mém. sur les Vers Intestinaux, 1858.

[81] Natural History Review, 1861, p. 44.

[82] Ann. des Sci. Nat. 1847, p. 90.

[83] Fauna littoralis Norvegiæ, pl. viii.

[84] Trans. of the Microsc. Soc. of London, 1851.

[85] Quarterly Journal of Microsc. Science, 1853.

[86] Wiegmann's Archiv., 1840, p. 196.

[87] Ueber die Erzeugung von Schnecken in Holothurier. Berlin, Bericht, 1851. Ann. Nat. Hist. 1852, v. ix. Müller's Archiv., 1852.

[88] Natürliche Schöpfungsgeschichte, pl. x.

[89] Ann. des Sci. Nat. 1853, p. 89.

[90] Ann. des Sci. Nat. 1857, pl. vi.

[91] Cyclopædia of Anatomy and Physiology. Art. Ovum, p. 4.

[92] Thomson, loc. cit. Article, Ovum, p. 139.

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