THE EVOLUTION OF ANIMALS. 381 



those of last year; they from those of the year before, 

 and so on back. 



2. All animals are subject to change. Offspring are 

 never just like the parents. If given time enough animals 

 may thus change in any degree. New species may come 

 from old in this way. 



3 . The animals of the present time are descended from 

 simpler, more generalized ones ; and these from still earlier 

 types. So even the most complex animals of the present 

 have arisen ultimately from ancestors as simple as the 

 simplest. 



4. All animals have fundamental likenesses. Some 

 are more alike; some are less so. The fundamental like- 

 nesses mean kinship. 



5. The process of life is gradual rather than sudden, 

 although the rapidity of it may differ at different times; 

 it is natural rather than supernatural; it is subject to the 

 same laws of cause and effect which operate in chemistry 

 and physics, and is not lawless and arbitrary. 



6. On the whole, the life-processes result in a closer and 

 more perfect adjustment of organisms to one another 

 and to the organic environment. 



379. Evidences for the Development Theory. Biolo- 

 gists generally are agreed as to the fact of evolution, and 

 there is no longer any direct search for proofs of the belief. 

 Any disagreement among them is in respect to the man- 

 ner in which evolution has come about, and the present 

 search is for the causes and the factors which produce it. 

 Many people, however, look with some suspicion on the 

 theory. For this reason the student should have before 

 his mind some of the classes of facts that have convinced 

 biologists of its truth. 



