Evolution of Animal Life. 141 



of animal life lias little to do with our present subject, which 

 is the operation of evolution in the animal kingdom, or, in 

 other words, the evolution of animal forms. This involves 

 more particularly the consideration of the Darwinian hypoth- 

 esis ; but, at the risk of seeming superfluously simple and 

 trite, I venture a prefatory explanation of the distinction 

 between Evolution and Darwinism. 



I. What is the Evolution of Animal Forms ? 



LeConte happily describes Evolution as "continuous, pro- 

 gressive change, according to certain laws and by means of 

 resident forces." As applied to animals, it means that all 

 existing forms, and all of which we have evidence from the 

 past, have been produced by descent with modifications from 

 pre-existing forms. 



The " laws " of this continuous change are merely formu- 

 las to express in general terms its observed facts. As given 

 by LeConte, they are : 



1. The law of differentiation, namely, the general fact 

 of a constantly increasing range of difference among exist- 

 ing forms. 



2. The law of the progress of the whole, namely, the gen- 

 eral fact that, although there is retrogression and reversion in 

 parts, the whole system steadily advances to higher func- 

 tions and wider variety, like a tree, the upward and spread- 

 ing growth of which as a whole is not measured by the 

 irregular form or deficient development or retrogressive 

 metamorphosis or death of any subordinate branch or leaf. 



3. The law of cyclical movement, namely, the wave-like, 

 successive domination of types, which rise, reach a maximum 

 and decline. 



These laws are not proofs of Evolution. Indeed, they 

 were chiefly established in their completeness by Agassiz, 

 the great opponent of that theory, who read in them mere- 

 ly the expression of the order in which successive forms 

 have been introduced. 



It is the third clause of LeConte 's definition, " by means 

 of resident forces" that characterizes the theory of Evolu- 

 tion. These resident forces are internal (determining he- 

 redity, variability, functional adaptability, etc.) or external 

 (the forces exercised by climate, supply of food, enemies 

 and rivals, etc.) The latter are summed up in the phrase, " the 

 environment." Evidently the forces which are internal to 



