DOCTRINE OF EVOLUTION AND RELATED IDEAS 483 



types. So even the most developed animals of the present have 

 arisen ultimately from ancestors as simple as the simplest. 

 Sometimes, however, evolution is regressive, from the complex 

 to the less complex. 



4. All animals have fundamental likenesses. Some are 

 more alike; some are less so. The fundamental likenesses mean 

 kinship. 



5. The process of the development 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 not lawless and 

 arbitrary, but is subject to the same laws of cause and effect 

 which operate in chemistry and physics. 



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

 perfect adjustment of organisms to one another and to the 

 more important forces of the 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 evidences for the belief. Any dis- 

 agreement among them is in respect to the manner in which 

 evolution has come about ; and the present search is for the cause 

 and the factors which produce it. Many people, however, look 

 with some suspicion on the idea. For this reason the student 

 should have before his mind some of the classes of facts that 

 have convinced biologists of the reality of evolution. 



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 variety has been brought about 

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

 isms themselves, by cultivation by man, or by something 

 belonging to 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. A proportion of 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 



