CHAPTER XXVII 



EVOLUTION 



THE student must not forget when discussing evolution that this 

 term means only that some present-day forms have become unlike 

 their ancestors, and that such difference is then transmittable — 

 in other words, that it has affected the germ-plasm. 



Evolution as it applies to the individual is, therefore, the name given 

 a process by, and through, which an organism evolves or changes into 

 a different type of being from its parents. 



In the chapter on genetics, we have seen how all offspring vary to 

 a small extent, not only from their parents, but from each other. When 

 such differences are slight, they are called variations and the organisms 

 possessing them are known as varieties. When such variations become 

 sufficient to set aside the new organism as a quite different type from 

 its parents, the new types are known as different species. 



It will be noted that this is quite vague ; for, what one man may 

 consider a difference sufficient to form a new species, another may not. 

 There is, therefore, no good definition of the term species. Biologists 

 disagree to a very marked extent as to what it means. 



Members of the. higher groups of animals are often considered as 

 belonging to the same species if they can inter-breed and give birth to 

 fertile offspring in turn. But, if we are to accept this definition, there 

 never can be any strictly new species ; for, if animals can inter-breed, 

 their offspring will belong to the species to which their parents belong, 

 and if they cannot, there will be no offspring. 



Then, there are those who take the position that only those indi- 

 viduals form true species which always breed true. If we accept this 

 definition, it may be said that whenever a so-called new-form or mutation 

 (as it is called) comes forth, such new form is in reality only the return 

 of some ancestral type, which has been formed by the meeting of an egg 

 and a sperm, both of which carried recessive characteristics. From this 

 angle one may always explain new species as being old ones, again 

 coming forth. 



However, species generally mean groups of individuals who possess 

 similar outstanding characteristics, which characteristics can be trans- 

 mitted to their offspring. 



First, then, in discussing evolution, one must be convinced that new 

 species really do come into existence, otherwise there can be no evolu- 

 tion. Practically all biologists now hold that new species do come into 

 being, which means that they accept evolution as a fact. There is, how- 



