« ПредыдущаяПродолжить »
arrangement. This gemmation, or, in other words, this stem structure, still occurs in Echinoderms, inasmuch as some species of star-fish possess such powers of reproduction as to enable a single arm or ray, when torn off, to complete itself into a whole animal. Nay, Kowalewsky's observations render it highly probable that the separation of rays, and their completion by gemmation, is in some species a normal process. Haeckel's hypothesis is thus laughed at only by those who are afraid to think
In the family of the Mollusca, the so-called navicula larva testifies the kinship of at least two of the great classes. The third and most advanced class, that of the cuttle-fish, had perhaps lost their distinctive badge even in those primæval times when, under the somewhat lower forms of the Tetrabranchiata, they left their shells in the Silurian strata. But the bivalve shells, or Lamellibranchiata, and the snails, widely differing in anatomical development, and constituting two natural classes, have a common larval form, or, if the larvæ display different shapes, a highly distinctive common larval organ, the velum. The accompanying diagram gives on the right the navicula of a cockle-shell as seen from behind. At the anterior end, two fleshy lobes have been formed, edged with cilia, by the vibrations of which the young animal, even in the egg, performs spiral twisting motions; in the midst of the cilia rises a little prominence, furnished with a longer filament. These ciliated lobes or vela, merging into one another, are shown on the left in the larva of a sea-snail (Pterotrachea), as seen nearly in profile, and in the phase in which the eyes and auditory apparatus, the foot and operculum, as well as a delicate
shell, have made their appearance. Here also, from the plane of the velum, a small fleshy protuberance juts out, without any special purport. The distribution of the velum, the period at which this larval organ makes its
appearance, its position towards the testa, head, mouth, and foot, and its subsequent effacement, one and all coincide exactly in the two classes. It is as yet of only a relatively small number of marine shells and slugs that we know the evolutionary history; yet we may infer that in these animals remaining in their original home, this heirloom has been generally preserved. Even genera which in their mature state scarcely recall the type of the Mollusca, as the boring mollusks (Dentalium Teredo), have preserved the phase of the navicula. On the other hand, in the branchiate fresh-water snails (Paludina) the velum is little developed, and in the land snails, which differ most widely from their marine kindred, the velum is entirely obliterated, as it is also among freshwater mussels. If in these animals adaptation and migration to land has had this effect on embryonic and post-embryonic development, we must suppose that in
the Cephalopoda, notwithstanding their continued sojourn in salt water, other causes have produced the loss of the velum phase, and the course of development peculiar to it.
With respect to the other fundamental forms of development, we may refer to the third chapter. The construction of the higher Articulata points to annulose progenitors, more or less corresponding to the annelids. of present times; and, again, the gradual increase of the segments of the larval annelids, which may be compared to the process of gemmation, leads from these higher Vermes to the lower ones with unsegmented bodies. All vertebrate animals, man included, if they do not preserve through life an unsegmented vertebral column, not separable into single vertebræ, are raised as embryos from this condition into their higher and definitive phase. That they should pass through this common embryonic condition is prohibited by all other mechanical causes but that of a common derivation from primordial forms which possessed an unsegmented vertebral column, no cranium or an imperfect one, and either no brain or one little differentiated from the spinal cord. Karl Ernst v. Baer, who, while we write these pages, raises his voice against the doctrine of Descent, has established the fact of types of development, and the course, within these types, from the undifferentiated to the special; but by the words "type of development,” the fact is paraphrased, not explained; and, as we cannot repeat too often, we prefer the distinct idea of derivation to the supposition of an unknown higher Power manifesting itself after an incomprehensible fashion in the types of development.
If the concatenation of the series by direct derivation and heredity be disallowed, it is absolutely inconceivable why the supreme creative Power, Nature, or the personal God, should have bound all higher animals to the same common stages of early development, and hereby exposed them to such manifold purposeless arrangements and great dangers. Of the millions of young oysters which annually escape from the egg, the majority perish under the disadvantages of external conditions, because the oyster has not yet divested itself of the ancient heirloom of the roving navicula. It has been able to compete successfully in the struggle for existence, only because, like most of its congeners, it is enormously prolific. This may be understood; but that a personal Creator, merely on principle, in order to keep the oyster within the type of development, should have endowed it with the phase of the navicula, in this case so extremely unpractical, can be accepted, like much other nonsense, only as matter of faith.
If accordance in the outlines of development has generally shown itself derivable from similarity of descent, we may now proceed to the explanation of those phenomena of development known to us as heterogenesis and metamorphosis. In these, the historical stages of development of whole classes and orders are inherited in the development of the individual; a proposition which is merely the corollary and application of what has been already intimated. In no class is there such a profusion of the phenomena of heterogenesis, readily submitting to explanation, as in that of the Medusa. We have already (p. 43) become acquainted with the origin of the Cladonema from the polype-like
Stauridium. The Medusa is the sexually mature form of the cycle of the species; its ova develope into polypes, which constitute the intermediate form in their development; that is to say, it is not transformed into the
animal from which it is derived, but produces gemmules. Only in this generation does the species revert to the sexual form.
We shall understand this alternation of generations. if we begin with the simplest Medusa polypes. Such a one is the annexed Hydractinea carnea, of which the female individual is portrayed. Compared with the intermediate form, Stauridium, the preliminary phase of the Cladonema, reproducing itself asexually,