IX.

The Development of the Individual (Ontogenesis) is a Repetition of the Historical Development of the Family (Phylogenesis).

ALTHOUGH the palæontological record is full of gaps, it is nevertheless unmistakable, as even most of the opponents of the doctrine of Descent are ready to admit, that from the older to the more recent period, a progress takes place from the lower to the higher grades of organisms, which is likewise exhibited in the system of the present vegetal and animal world; and that in many ways embryonic development as well as metamorphosis and heterogenesis,--in a word, individual development ("Ontogenesis," Haeckel) suggests a comparison with these palæontological series, as well as with the systematic order of succession. The parallelism of the palæontological and the systematic series is either a miracle, or it may be accounted for by the doctrine of Descent. There is no other alternative. And the doctrine of Descent fully bears the test; it shows how the derivation of the present organisms from those previously existing rests on the transmission of the characters of the progenitors to the offspring and the acquisitions of the individuals. The phenomena of individual development or Ontogenesis admit of no other choice; either they remain uncomprehensible, or they

stand the test of the doctrine of Descent and submit to the great general principle.

If we scrutinize the countless facts of reproduction and development, they certainly admit of classification; they range themselves in analogous and homologous groups; types of development become apparent; we speak of development without metamorphosis, of transformation, and heterogenesis. But what necessary relation the alternating forms, the shapes appearing in heterogenesis, bear to the complete animal or the sexually developed chief representative of the species? why so many animals undergo no transformations, but emerge "complete" from the egg? why the species belonging to the same class or "type" possess the same type of development and process of construction ?-these and similar questions as to the interpretation of the tangled mass of facts press themselves upon us. And they are also tests of our theory of derivation. The doctrine does as much as has been done by any great hypothesis in its special application; and if it gives a satisfactory reply to all, or at least to nearly all, pertinent questions, these are so many witnesses and proofs of its truth, which, according to all scientific custom and justice and philosophic method, will remain valid until the falsity of the inductions and inferences has been demonstrated and a better hypothesis substituted in its stead.

The first proposition derived from the doctrine of Descent in explanation of the facts of individual development may run thus: accordance in the outlines. of development is based on similar derivation; or, somewhat differently stated: accordance in the outlines of individual development is accounted for by

DEVELOPMENT OF ECHINODERMS.

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similarity of derivation. As we already know, C. E. v. Baer first demonstrated that the members of the great divisions of the animal kingdom agreeing in the outlines of their organization testify their coherence by a special "type of development." This fact was always looked upon as self-evident, although, if it were not derived from descent, it would be the greatest miracle. This is therefore the place for us to review some of the fundamental forms of development which we partially considered in the third chapter, and at the same time to elucidate the meaning of these types with the aid of the doctrine of derivation.

We will take the Echinoderm as our first example. Although from the anatomical comparison of a crinoid, a star-fish, a sea-urchin, and a sea-cucumber or holothuria, the close kindred of these various divisions of echinoderms is easily deduced, they yet deviate wonderfully from one another in outward shape and in the construction of the skeleton. The relative value of the difference between a holothuria and a star-fish, a sea-urchin and a comatula, may be compared to the difference between a mammal and a bird, an amphibian and a fish. Nevertheless, with some few exceptions which have a special meaning, these various echinoderms leave the egg in a larval state almost identical. The larva (Fig. 12) is boat-like in form, with a curved mar

gin bent over at both ends like a deck. This border is edged with a continuous row of cilia, by the agency of which the little boat is moved. A short digestive canal, provided with a gastric enlargement, is the first essential organ of this

FIG. 12.

body. We will not describe the highly complex transformations of the larva here into an ophiura, there into a sea-urchin, and here again into a sea-cucumber; but we will only inquire what can be the cause of this accordance in the earliest stages of individual development. There is no reasonable answer but the derivation of all echinoderms known to us from an older form, in the development of which our larva likewise appeared, and from which this common phase of development was transmitted to the whole family. But it is allowable to ask, further, how from a bilateral larva, one, that is, symmetric on two sides, should be evolved in animals of radiate structure, as are the greater number of mature echinoderms?

On this point Haeckel instituted a conjecture which at first exasperated the systematizers of the old school, but which now gains more and more footing, and is supported by the most recent comparative investigations, such as those of Hoffmann "On the Minute Anatomy of the Starfish" ("Ueber die feinere Anatomie der See-sterne"). The boat-shaped larva of the Echinoderms, especially a modification occurring in the starfish, strikingly resembles a certain larval type of the marine Annelida. And as in the structure and distribution of the parts of the rays of the echinoderms, especially of the star-fish, an unmistakable resemblance with the relative distribution and succession of parts of the Annelids is observable, Haeckel regards the Echinoderms as an offshoot of the Annelids. He considers that the oldest, and to us unknown, echinoderms originated as annelid stems; the anterior end of the bilateral annulose parent-animal budding out gemmules in a radiate

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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

or reason.

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