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 _Magosphaera_. From the minute cells (Fig. 63) arising through this process of yolk-segmentation, the body of the Tardigrade is then built up.[78]
[Illustration: FIG. 60, Egg of Tardigrade, Kaufmann, Zeit f. Wiss.
Zool. 1851, Pl. 1. 61, Egg of Tardigrade after the yolk has subdivided.
62, Egg of Tardigrade in the next stage. 63, Egg of Tardigrade more advanced.]
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 _Magosphaera_ 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 _Magosphaera_, 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 _Sphaerularia bombi_.[81]
Among the Echinoderms M. Derbes thus describes the first stages (Pl. 6, Figs. 10-13) in the development of the egg of an _Echinus_ (_Echinus esculentus_):-"Le jaune commence a se segmenter, d'abord en deux, puis en quatre et ainsi de suite, chacune des nouvelles cellules se partageant a son tour en deux."[82] Sars has observed the same thing in the starfish.[83]
[Illustration: PLATE. 6.]
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_), Muller 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 larvae closely resembling _Campodea_.
Since, then, individual insects are certainly in many cases developed from larvae 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 larvae 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 _Chloeon_."
[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., ser. 4, tome vii. See also _Natural History Review_, April 1862.
[13] Ann. and Mag. of Nat. Hist. 1852.
[14] Zeits. fur Wiss. Zool. 1869.
[15] Transactions of the Linnean Society, 1863.
[16] Lectures on the Anatomy, &c. of the Invertebrate Animals.
[17] Untersuchungen uber 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] Mem. de l'Acad. Imp. des Sci. de St. Petersbourg. 1869.
[21] Observationes de Prima Insectorum Genesi, p. 14.
[22] Mem. de l'Acad. Imp. des Sci. de St. Petersbourg. tome xvi. 1871, p. 35.
[23] Recherches sur l'Evolution des Araignees.
[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. Kon.