FOOTNOTES:

PLATE V.PLATE V.Figs. 1-5,Protamœba6-9,Protamyxa aurantiaca, Haeckel, Beit. zur Monog. der Moneren, pl. 1; 10-18,Magosphœra planula, Haeckel, loc. cit. pl. 5.]

PLATE V.

Figs. 1-5,Protamœba6-9,Protamyxa aurantiaca, Haeckel, Beit. zur Monog. der Moneren, pl. 1; 10-18,Magosphœra planula, Haeckel, loc. cit. pl. 5.]

In the trueAmœbas, 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 theAmœbas, as inProtamœba, 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 theProtomyxa aurantiaca(Pl.V, Fig. 8), discovered by Haeckel76on dead shells ofSpirula, where it appears as a minute orange speck, which shows well against the clear white of theSpirula. 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.V, Fig. 9), after which the containing membrane burst, and the separate spherules, losing their globular form, crept out as small Amœbæ (Pl.V, Fig. 6), or amœboid 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.V, 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 theMagosphæra planula, discovered by Haeckel on the coast of Norway.

In one stage of its existence (Pl.V, Fig. 10) it is a minute mass of gelatinous matter, which continually alters its form, moves about, feeds, and in fact behaves altogether like theAmœbajust described. It does not, however, remain always in this condition. After a while it contracts into a spherical form (Pl.V, 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.V, Fig. 12); these two subdivide into four (Pl.V, 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.V, Fig. 15). Here this process ends. The separate spherules now begin to lose their smooth outline, to throw out processes, and to show amœboid 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 sphericalMagosphæra, 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, whichthen swims about freely in the water (Pl.V, Fig. 16), thus closely resemblingSynura, 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.V, 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 Amœbæ (Pl.V, Fig. 18). Finally, according to Haeckel, these amœboid 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 instanceGregarina, is of a similar character.

It may be said, and said truly, that the difference between such beings as these and theCampodea, 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.77As shown in Fig.60, the egg is at first a round body or cell, with a clear central nucleus—the germinalvesicle; 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 inMagosphæra. From the minute cells (Fig.63) arising through this process of yolk-segmentation, the body of the Tardigrade is then built up.78

Figs. 60-63Fig. 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.

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 ofMagosphœrawith 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 describes79the process inLaomedea, as representing the Hydroids (Pl.VI, Fig. 1, represents the young egg):—"The first step observable in the segmentation-process is thecleavage of the yolk into two segments (Pl.VI, Fig. 2), immediately followed by the cleavage of these into other two, so that the vitellus is now composed of four cleavage spheres (Pl.VI, Fig. 3)." These spheres again divide (Pl.VI, Fig. 4) and subdivide, thus at length forming minute cells, of which the body of the embryo is built up.

In Pl.VI, Figs. 5-9 represent the corresponding stages in the development of a small parasitic worm—theFilaria mustelarum—as given by Van Beneden.80The first process is that within the egg, which represents, so to say, the encysted condition ofMagosphœra, the yolk divides itself into two balls (Pl.VI, 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 abnormalSphærularia bombi.81

Among the Echinoderms M. Derbès thus describes the first stages (Pl.VI, Figs. 10-13) in the development of the egg of anEchinus(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."82Sars has observed the same thing in the star-fish.83

PLATE VI.PLATE. 6.

PLATE. 6.

In the Rotatoria, as shown by Huxley inLacinularia,84and by Williamson inMelicerta,85the yolk is atfirst 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 mollusca86(Tritonia), Müller in Entochocha,87Haeckel in Ascidia,88Lacaze Duthiers inDentalium.89Figures 18 to 21, Pl.VI, are taken from Koren and Danielssen’s90memoir on the development ofPurpura lapillus.

Figs.22-24show 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,91as 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 advancedgroups; 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 resemblingCampodea.

Since, then, individual insects are certainly in many cases developed from larvæ closely resembling the genusCampodea, 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 fewdays, is surely no wild or extravagant hypothesis. Again, other insects come from vermiform larvæ much resembling the genusLindia, 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 genusCampodea, and that these again have arisen from others belonging to a type represented more or less closely by the existing genusLindia.

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.

1Darwin’s “Researches into the Geology and Natural History of the Countries visited by H.M.S.Beagle,” p. 326.

2Introduction to Entomology, vi. p. 50.

3Manual of Entomology, p. 30.

4Linnean Journal, vol. xi.

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

6Linnean Transactions, 1863—"On the Development ofChloëon."

7The 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 desClavigerspar les Fourmis.” Bull. de la Soc. d’Anthropologie de Paris, 1868, p. 315.

9Westwood’s Introduction, vol. i. p. 36.

10Westwood’s Introduction, vol. ii. p. 52.

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

12Ann. des Sci. Nat., sér. 4, tome vii. See alsoNatural History Review, April 1862.

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

14Zeits. für Wiss. Zool. 1869.

15Transactions of the Linnean Society, 1863.

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

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

18Linnean Transactions, vol. xxii. 1858.

19“Embryological Studies on Hexapodous Insects.” Peabody Academy of Science. Third Memoir.

20Mém. de l’Acad. Imp. des Sci. de St. Pétersbourg. 1869.

21Observationes de Prima Insectorum Genesi, p. 14.

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

23Recherches sur l’Evolution des Araignées.

24Philosophical Transactions, 1841.

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

26Loc. cit. p. 315.

27Philosophical Transactions, 1859, p. 589.

28“Facts for Darwin,” Eng. Trans. p. 127.

29Rolleston, “Forms of Animal Life,” p. 146.

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

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

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

33Philosophical Transactions, 1865 and 1866.

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

35Thomson, on the Embryology of the Echinodermata,Natural History Review, 1863, p. 415. See also Agassiz, “Embryology of the Starfish,” p. 62.

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

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

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

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

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

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

42Principles of Biology, vi. p. 349.

43For differences in larva consequent on variation in the external condition, seeante, p. 61.

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

45See Newport, Phil. Trans., 1832.

46Linnean Transactions, 1862.

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

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

49Zeit. für Wiss. Zool. 1863.

50Mém. de l’Acad. Imp. de St. Pétersbourg. vol. xv. 1870.

51Of 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.”—Huxleyon Animal Individuality, Ann. and Mag. of Nat. Hist. June 1852.

53Prince Hohenstiel Schwangau, p. 68.

54Journal of the Royal Institution. April 1873.

55“Embryology of Echinoderms,” l. c. p. 15.

56Mr. and Mrs. Agassiz: “Seaside Studies,” p. 139.

57l. c. p. 138.

58Wien. Zool. Bot. Gesells, 1869.

59Linnean Transactions, 1863.

60Linnean Transactions, 1866, vol. xxv.

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

62Siebold und Kolliker’s Zeitschr. f. Wiss. Zool., 1864.

63Linnean Journal, vol. xi.

64Facts for Darwin, p. 120.

65A still nearer approach is afforded by the genusPeripatus, 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.

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

67Linnean Transactions, v. xxii.

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

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

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

71See 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 genusEchinoderes, 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.

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

74Monographie der Moneren, p. 43.

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

76Monographieder Moneren, p. 10.

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

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

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

80Mém. sur les Vers Intestinaux, 1858.

81Natural History Review, 1861, p. 44.

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

83Fauna littoralis Norvegiæ, pl. viii.

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

85Quarterly Journal of Microsc. Science, 1853.

86Wiegmann’s Archiv., 1840, p. 196.

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

88Natürliche Schöpfungsgeschichte, pl. x.

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

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

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

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

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