REVIEW OF THE MODERN DOCTRINE OF EVOLUTION. 233 



Paleontology shows that the succession of living types has not 

 been in a. single straight line. It has been in many divergent lines, 

 and a large number of them have not continued to the present 

 time. The history of life has been well compared to a tree with 

 divergent branches, many of which do not reach the elevation of 

 the summit. Furthermore, in the many cases in which we can 

 trace the lower lines to the present period, it is evident -that in 

 - their present condition they could not have given rise to the higher 

 forms. Each line, in fact, has developed to an extreme of spe- 

 cialization of structure, which it would seem is incapable of modifi- 

 cation in any direction very divergent from that which it has al- 

 ready taken. Much less have such specialized types been able to 

 survive the environment for which they were designed ; with im- 

 portant changes in that respect they have perished. A few exam- 

 ples will serve to illustrate my meaning. The direction of develop- 

 ment has been from fishes, through BatracMa and rei)tiles, to birds 

 and mammals. But we can not derive any living type from the 

 osseous fishes of the present or past ages {Teleostomi): to find the 

 origin of BatracMa, we must pass below these to more generalized 

 and older forms, the Dipnoi, a class whose position in the system 

 was for years a controverted point. We can not obtain Mammalia 

 from any of the existing types of reptiles, but we must go back to 

 the Permian period, and trace their outlines in the Tlieromorplia 

 of that day. In spite of the prophetic resemblance of these 

 remarkable animals, they are inferior to later Eeptilia in the 

 structure of their vertebral column, and display resemblance to 

 some of their immature stages, as well as to those of the Mammalia. 

 Among mammals we can not derive monkeys from Carnivora or 

 Ungulata, nor the latter from each other, but can only trace their 

 close approximation in the Bunotherian types of the Lower Eocene. 

 So with the great divisions of Ungulata ; Prohoscidians, Hyrax, 

 and the even- and odd-toed orders must all be traced to the un- 

 specialized AmUijpoda, with small brains and five-toed plantigrade 

 feet, as their ancestors.* It is easy to perceive that the generali- 

 zation and plasticity of all these forms has furnished the ground 

 of their ancestral relation. 



"We are now in a position to comprehend more clearly the 

 general nature of evolution. The doctrine of the unspecialized 



* See the origin of types of Mammalia educabilia, " Journal Academy," Philadel- 

 phia, 1874. 



