234 POPULAR SCIENCE MONTHLY. 



and dolphins; still more it moulds similar and larger groups of animals 

 into similar lines or radii of specialization. Thus we reach the grand 

 idea of analogy as operating in the divergencies or adaptive radiations 

 of groups, according to which great orders of animals tend in their 

 families and suborders to minic other orders, and the faunae or col- 

 lective orders of continents to mimic the faunae of other continents. 



Amid this repetition on a grand scale of similar adaptations, which 

 is altogether comparable to what we know as having occurred over and 

 over again in human history, the paleontologist as a historian must 

 keep constantly before him the second great idea of homogeny, of real 

 .•ancestral kinship, of direct blood descent and hereditary relationship. 

 'The shark and the ichthyosaur superficially look alike, but their germ 

 ■cells are radically different, their external resemblances are a mere 

 veneer of adaptation so deceptive, however, that it may be a matter of 

 half a century before we recognize the wolf beneath the clothing of the 

 sheep, or the ass in the lion's skin. 



These two great ideas of analogy or similarity of habit, and homog- 

 eny or similarity of descent, do not run on the same lines; they are 

 the woof and the warp of animal history. Analogy corresponds to 

 the woof or horizontal strands which tie animals together by their 

 superficial resemblances in the present, homogenies are the warp, or 

 the fundamental vertical strands which connect animals with their 

 ancestors and their successors. The far reaching extent of analogous 



revolution was only dimly perceived by Huxley, and constituted his one 

 great defect as a philosophical anatomist. Its power of transforming 

 unlike and unrelated animals has accomplished miracles in the way of 

 producing a likeness so exact that the inference of kinship is almost 

 irresistible. 



The paleontologist who would succeed as historian must first, there- 

 fore, render himself immune to the misguiding influences of analogy 

 by taking certain further precautions which will now be explained by 

 watching his procedure as historian. 



Paleontology as the history of life takes its place in the back- 

 ground of recorded history and archeology, and simply from the stand- 

 point of the human pedigree is of transcendent interest. Although it 

 has progressed far beyond the dreams of Darwin and Huxley, the first 

 general statement which must be made is that the actual points of con- 

 tact between the grand divisions of the animal and plant kingdom, as 

 well as between the lesser and even many of the minor divisions, have 

 yet to be discovered. You recall that the older grand divisions of the 

 Vertebrata, to which we must confine our attention, were suggested by 

 the so-called Ages of Fishes, of Amphibians, of Reptiles and of Mam- 

 mals. Even within these grand divisions we observe a succession of 

 more or less closely analogous groups. Each of these groups has its 



