On the Origin and Metamorphoses of Insects

In the true Amœbas, 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 Amœbas, as in Protamœba, 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. V, Fig. 8), discovered by Haeckel76 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. 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 the Magosphæra 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 the Amœba just 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 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. V, 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. 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 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

Among the Zoophytes Prof. Allman thus describes79 the process in Laomedea, as representing the Hydroids (Pl. VI, Fig. 1, represents the young egg):—"The first step observable in the segmentation-process is the cleavage 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—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 Magosphœra, 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 abnormal Sphærularia bombi.81

Among the Echinoderms M. Derbès thus describes the first stages (Pl. VI, 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 star-fish.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 mollusca86 (Tritonia), Müller in Entochocha,87 Haeckel in Ascidia,88 Lacaze Duthiers in Dentalium.89 Figures 18 to 21, Pl. VI, are taken from Koren and Danielssen’s90 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:

THE END.

RICHARD CLAY AND SONS, LIMITED, LONDON AND BUNGAY.

BY THE SAME AUTHOR.

NATURE SERIES.