Page:Popular Science Monthly Volume 66.djvu/240

236 have been written by embryologists and comparative anatomists on the hypothetical transformation of the fin into the hand. Considering the supreme value of the hand and foot in vertebrate history, this was certainly the most momentous transformation of all and worthy of volumes of speculation; but as a matter of fact, the speculation has been a total failure, and this problem of problems will only be settled by the future discovery in Devonian rocks of the actual connecting link, which will be a partly air-breathing fish, capable of emerging upon land, in which the cartilages of the fin will be found disposed very much as in the limbs of the earliest Carboniferous amphibians. The unity of composition in the hand and the foot points to an original similarity of habit in the use of these organs.

This missing point of contact, or of the actual link between amphibians and fishes, is equally characteristic of paleontology as history from the top to the bottom of the animal scale. We are positive that Amphibians descended from fishes, probably of the crossopterygian kind, but the link still eludes us; we have brought the reptiles within close reach of the amphibians, but the direct link is still to be found; mammals are in close proximity to a certain order of reptiles, but the connecting form is still undiscovered; man himself is not far from the various types of anthropoid apes, but his actual connecting relationship is unknown.

We are no longer content, however, with these approaches to actual contact and genetic kinship, we have toiled so long both by discovery and by the elimination of one error after another, and are so near the promised land, we can hardly restrain our impatience. I venture to predict that the contact of the Amphibia with the fishes will be found either in America or Europe. No such prediction could be safely made regarding the connecting form between the amphibians and reptiles, because America, Eurasia and Africa all show in contemporaneous deposits evidence that such connection may be discovered at any time. The transformation from reptiles to birds will probably be -found in the Permian of America or Eurasia; chances of connecting the mammals with the reptiles are decidedly brightest in South Africa; while in Europe, or more probably in Asia, we shall connect man with generalized catarhine primates.

Passing from these larger questions of the relations of the great classes of vertebrates to each other, let us review the problems arising in the individual evolution of the classes themselves.

The primordial, solid-skulled or stegocephalian amphibia of the Permian diverged into a great variety of forms which wandered over Eurasia and North America so freely that, for example, we find as close a resemblance between certain Würtemberg and New Mexican genera