XIL] "THE ORIGIN OF SPECIES. 315 



The doctrine of evolution in biology is the neces- 

 sary result of the logical application of the principles 

 of uniformitarianism to the phenomena of life. Dar- 

 win is the natural successor of Button and Lyell, and 

 the "Origin of Species" the logical sequence of the 

 " Principles of Geology." 



The fundamental doctrine of the "Origin of 

 Species," as of all forms of the theory of evolution 

 applied to biology, is " that the innumerable species, 

 genera, and families of organic beings with which the 

 world is peopled have all descended, each within its 

 own class or group, from common parents, and have 

 all been modified in the course of descent." l 



And, in view of the facts of geology, it follows 

 that all living animals and plants " are the lineal 

 descendants of those which lived long before the 

 Silurian epoch." 2 



It is an obvious consequence of this theory of 

 descent with modification, as it is sometimes called, 

 that all plants and animals, however different they 

 may now be, must, at one time or other, have been 

 connected by direct or indirect intermediate grada- 

 tions, and that the appearance of isolation presented 

 by various groups of organic beings must be unreal. 



No part of Mr. Darwin's work ran more directly 

 counter to the prepossessions of naturalists twenty 

 years ago than this. And such prepossessions were 

 very excusable, for there was undoubtedly a great 

 deal to be said, at that time, in favour of the fixity of 



1 " Origin of Species," ed. 1, p. 457. 

 2 Ibid. p. 458. 



