io MENDELISM CHAP. 



some of their offspring. A competitive struggle for 

 existence working in combination with certain 

 principles of variation and heredity results in a slow 

 and continuous transformation of species through 

 the operation of a process which Darwin termed 

 natural selection. 



The coherence and simplicity of the theory, sup- 

 ported as it was by the great array of facts which 

 Darwin had patiently marshalled together, rapidly 

 gained the enthusiastic support of the great majority 

 of biologists. The problem of the relation of species 

 at last appeared to be solved, and for the next forty 

 years zoologists and botanists were busily engaged 

 in classifying, by the light of Darwin's theory, the 

 great masses of anatomical facts which had already 

 accumulated, and in adding and classifying fresh ones. 

 The study of comparative anatomy and embryology 

 received a new stimulus, for with the acceptance of 

 the theory of descent with modification it became 

 incumbent upon the biologist to demonstrate the 

 manner in which animals and plants differing widely 

 in structure and appearance could be conceivably 

 related to one another. Thenceforward the energies 

 of both botanists and zoologists have been devoted 

 to the construction of hypothetical pedigrees suggest- 

 ing the various tracks of evolution by which one 

 group of animals or plants may have arisen from 

 another through a long-continued process of natural 

 selection. The result of such work on the whole 

 may be said to have shown that the diverse forms 

 under which living things exist to-day, and have 

 existed in the past so far as palaeontology can tell 

 us, are consistent with the view that they are all 



