DOCTRINE OF EVOLUTION AND RELATED IDEAS 477 



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 likenesses 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 inor- 

 ganic environment. 



480. Evidences for the Development Theory. Biologists 

 generally are agreed as to the fact of evolution, and there is no 

 longer any direct search for proofs of the belief. Any disagree- 

 ment among them is in respect to the manner 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. 



481. Variability as an Evidence. The changeableness of 

 organisms is the fact that makes it impossible for the biologist 

 to deny evolution. Every day we see differences in organisms 

 of the same species, which differences have been brought about 

 by differences in the surroundings, by the behavior of the organ- 

 isms .themselves, by cultivation by man, or by something 

 inherited from their parents. We know that man can take ad- 

 vantage of these differences and can select certain types; can 

 cultivate and select again in such a way as to get, in a few gen- 

 erations of cultivation and breeding, animals strikingly different 

 from those with which he started. After a certain time these 

 new forms seem to breed reasonably true, and a new race is said 

 to be established. In this way the different breeds or varieties 

 of dogs, pigeons, chickens, and many other domestic animals 

 have apparently arisen. This is not merely a proof of evolution; 



