PRESENT PROBLEMS 575 



radiations of groups, according to which great orders of animals tend 

 in their families and suborders to mimic other orders, and the faunae 

 or collective orders of continents to mimic the faunae of other con- 

 tinents. 



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 an historian 

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

 of real ancestral kinship, of direct blood descent and hereditary rela- 

 tionship. 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 homo- 

 geny, 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; homogeny to the warp, or 

 the fundamental vertical strands which connect animals with their 

 ancestors and their successors. The far-reaching extent of analogous 

 evolution was only dimly perceived by Huxley, and this fact con- 

 stituted 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 infer- 

 ence 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 background 

 of recorded history and archeology, and simply from the standpoint 

 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 

 contact 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 sug- 

 gested by the so-called Ages of Fishes, of Amphibians, of Reptiles, and 

 of Mammals. Even within these grand divisions we observe a succes- 

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

 its more or less central starting-point in a smaller and older group 

 which contains a large number of primitive or generalized characters. 



The search for the primitive central form is always made by the 



