358 PRINCIPLES OF ANIMAL BIOLOGY 



Paleontology furnishes perhaps the best evidence of the course of evolu- 

 tion when the series of discovered fossils is fairly complete. But evidence 

 as good as that concerning the horses and elephants is not common. To 

 trace the lines of descent of animals whose ancestors are not abundantly 

 represented among known fossils, reliance is placed on comparative 

 anatomy, comparative embryology, and geographical distribution. In 

 erecting a family tree upon the basis of facts furnished by these sciences 

 there is, however, much uncertainty. It is not difficult, in most cases, 

 to decide what animals have descended from a common ancestor. When 

 two species of garter snake differ only or principally in the number of 

 rows of scales, they may safely be regarded as having been produced 

 from the same stock. But was the ancestral species like the one modern 

 species, or like the other? Or was it different from both? In answering 

 such questions the zoologist applies certain rules which seem to him to 

 express probabilities. These rules may depend upon the amount of 

 variability in the character which distinguishes the present species, or the 

 present distribution of these and other similar species. But any con- 

 clusion based on such rules is subject to risk, and the merits of each case 

 have to be decided separately. In tracing the descent of the larger groups, 

 still greater uncertainty exists. As pointed out above, the vertebrates 

 have plainly come from a gill-bearing animal. But what are the steps 

 by which the present condition, especially in those classes in which gills 

 no longer occur (reptiles, birds, mammals), has been attained? Here 

 opinion differs, and there has been much controversy over questions 

 concerning the mutual relationships and intermediate stages of descent 

 of these groups. When the biologist grows still bolder, and attempts 

 to go back of the ancestors of the phylum, tracing the course of evolution 

 is practically without foundation. It has been held probable, for ex- 

 ample, that the annelids and arthropods have descended from common 

 ancestors, partly because the members of both these phyla are segmented. 

 But of the steps in the divergence of the two groups, if they really are 

 related, there is almost no evidence whatever. Tracing such supposed 

 pedigrees was fashionable among zoologists during the latter third of 

 the Nineteenth century, until it was realized how speculative is the 

 process. 



Factors Directing the Course of Evolution. — Ideas regarding the 

 agencies that guide evolution are as old as ideas of evolution itself. 

 Speculation as to the reasons for the development of lines of descent is 

 really older than the attempt to build family trees. Evolution was earl}'- 

 thought of as a process of becoming perfect, whatever that might mean; 

 and before this idea became concrete, and took the form of specific lines 

 of descent, there was conjecture as to what led animals toward the goal 

 of perfection. 



