4 EVOLUTION AND ANIMAL LIFE 



development. That species do change their structure with 

 time or with space is a matter of common scientific observation. 

 With the lapse of time, generation following generation, direct- 

 ive influences combine to modify the line of descent. With 

 the separation of individuals by barriers of land and water 

 and varying climate, differing lines of descent are brought into 

 existence. The fact of descent with modification large or small 

 is a matter of common knowledge in the biology of to-day, veri- 

 fied in the hundreds of thousands of species of organisms now 

 known and classified. To call this transmutation of species is 

 but to state the fact. To call it evolution is to suggest a theory 

 that all these changes are but the unrolling of the plan a move- 

 ment toward some predetermined end. That this is true we have 

 no means of knowing, and the results as they appear to us seem 

 to be determined by proximate causes alone. Among these 

 proximate causes are differences in structure and in degrees of 

 adaptability among individuals, the operation of the rule of 

 the survival of the best adapted, the inheritance by individuals 

 of the traits of the immediate ancestry, and the effects of cli- 

 matic changes, and of migrations hampered and unhampered 

 by the presence of physical barriers. The effects of influ- 

 ences like these are considered by most writers as the es- 

 sential elements in "organic evolution." But a few writers 

 give external influences a secondary place, confining the term 

 evolution solely to the results of causes resident within the 

 individual. 



Speaking broadly we find as a fact that transmutation of 

 species through the geologic ages has been accompanied by 

 increasing divergence of type, by the increased specialization 

 of certain forms, and by the closer and closer adaptation to 

 conditions of life on the part of the forms most highly special- 

 ized, the more perfect adaptation and the more elaborate 

 specialization being associated with the greatest variety or 

 variation in environment. Accepting for this process the name 

 of organic evolution, Herbert Spencer has deduced from it the 

 general law that as life endures generation after generation, its 

 character, as shown in structure and function, undergoes con- 

 stant differentiation and specialization. In this view, the 

 transmutation of species is not merely an observed process, but 

 a primitive necessity involved in the very organization of life 

 itself. 



