126 GEOLOGICAL BIOLOGY. 



This school, which Forbes represents, assigned all the 

 steps of progress observed in the history of organisms to 

 causes entirely antecedent to each individual's birth. The 

 explanation was confined to ancestry in its abstract sense of 

 "antecessor" (as the Latin original has it): all cause of 

 changes of a specific rank was entirely antecedent to the 

 organic individual expressing them. The fundamental charac- 

 teristic of this view is found in the doctrine of the " immuta- 

 bility of species," as contrasted with the doctrine of " muta- 

 bility of species " of the new school. 



Mutability of Species the Central Thought in the New Theory 

 of the Origin of Species. — Nothing that has occurred in the 

 present century has so stimulated investigation of the facts of 

 nature, and has so pervaded the whole realm of philosophical 

 thought, as that which has centred about this question as to 

 the nature and origin of the organic species. Darwin's 

 famous work " The Origin of Species," first published in 

 November, 1859, struck the key-note of the present age of 

 the science. He clearly announced the opinion that species 

 are mutable, and as the whole science of natural history was 

 built on the idea of their immutability, a complete readjust- 

 ment of the science to the new conception has resulted. 

 The importance of a clear conception of the meaning of 

 species is thus apparent, and it will be discussed in detail in a 

 following chapter. The idea of immutability of species ob- 

 structed the way to the clear comprehension of the evolution of 

 organisms, very much as the catastrophe theory of the end of 

 the last century prevented geologists from reaching a clear 

 understanding of the agencies and methods by which the 

 earth reached its present condition. Uniformitarianism 

 played much the same role for Geology which evolutionism is 

 working for the science of Biology. 



Two Extremes of Opinion Regarding the Mode of Origin of 

 Species by Evolution. — Among those to-day who adopt evolu- 

 tion as the explanation of the mode of origin of the different 

 forms of organisms, there are two extremes of opinion with 

 many intermediate compromises. 



All will agree in recognizing ancestry and environment as 

 each taking some part in the evolution ; but the extreme 



