Page:Popular Science Monthly Volume 61.djvu/542

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NE of the most interesting problems in vertebrate morphology, and one of the most important from its wide-reaching relations, is that of the derivation of the fins of fishes. This resolves itself at once into two problems, the origin of the median fins, which appear in the lancelets, at the very bottom of the fish-like series, and the origin of the paired fins or limbs, which are much more complex, and which first appear with the primitive sharks.

In this study the problem is to ascertain not what theoretically should happen, but what, as a matter of fact, has happened in the early history of the fish-like groups. That these structures, with the others in the fish body, have sprung from simple origins, growing more complex with the demands of varied conditions, and then at times again simple, through degeneration, there can be no doubt. It is also certain that each structure must have had some element of usefulness in all its stages. In such studies we have, as Haeckel has expressed it, 'three ancestral documents, paleontology, morphology and ontogeny,'—the actual history as shown by fossil remains, the side-light derived from comparison of structures, and the evidence of the hereditary influences shown in the development of the individual. As to the first of these ancestral documents, the evidence of paleontology is conclusive where it is complete. But in very few cases are we sure of any series of details. The records of geology are like a book with half its leaves torn out, the other half confused, displaced and blotted.

The evidence of comparative anatomy is most completely secured, but it is often indecisive as to relative age and primitiveness of origin among structures. As to ontogeny, it is, of course, true that through heredity, 'the life history of the individual is an epitome of the life history of the race.' 'Ontogeny repeats phytogeny,' and phylogeny, or line of descent of organisms and structures, is what we are seeking. But here the repetition is never perfect, never so perfect in fact as Haeckel and his followers expected to find it. The demands of natural selection may lead to the lengthening, shortening, or distortion of phases of growth, just as they may modify adult conditions. The conditions of the individual development may, therefore, furnish evidence in favor of certain theories of origins, but they cannot alone furnish the absolute proof.