232 THE COMING OF AGE OF VII 
day. No physical geologist now dreams of seeking, 
outside the range of known natural causes, for the 
explanation of anything that happened millions of 
years ago, any more than he would be guilty 
of the like absurdity in regard to current events. 
The effect of this change of opinion upon biolo- 
gical speculation is obvious. For, if there have 
been no periodical general physical catastrophes, 
what brought about the assumed general ex- 
tinctions and re-creations of life which are the 
corresponding biological catastrophes? And, if no 
such interruptions of the ordinary course of nature 
have taken place in the organic, any more than in 
the imorganic, world, what alternative is there to 
the admission of evolution ? 
The doctrine of evolution in biology is the 
necessary result of the logical application of the 
principles of uniformitarianism to the phenomena 
of life. Darwin is the natural successor of Hutton — 
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.” + | | 
1 Origin of Species, ed. 1, p. 457. 
