236 POPULAR SCIENCE MONTHLY. 



have been written by embryo-logists and comparative anatomists on 

 the hypothetical transformation of the fin into the hand. Consider- 

 ing 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 his- 

 tory 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 dis- 

 covery 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 contem- 

 poraneous 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. 



Geographical Problems. 



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 Wurtemberg and New Mexican genera 



