i4 GENERAL riilNCIPLES OF ZOOLOGY. 



imation to the goal, since lie uses for breeding only the suitable 

 individuals from each new generation. If he wish, for example, 

 to breed fautail pigeons, he selects from his stock animals with 

 the most numerous and strongest tail-feathers. In the course of 

 generations, then, characteristics cumulate; the number of jngeons 

 having an increased number of tail-feathers becomes greater, and 

 thus material is obtained which is adapted to a further increase in 

 the number of feathers. 



Factors of Evolution in Breeding. — The remarkable results of 

 breeding which are well known to every observer of our domesti- 

 cated animals depend mainly upon three factors : (1) Variability: 

 the descendants of one pair of parents have the caj^ability of 

 developing new characteristics, thereby differing in appearance 

 from their parents. (2) Hereditalility of newlj'-acquired charac- 

 ters. This consists in the tendency of the daughter-generation to 

 transmit the newly-developed characteristic to the succeeding 

 generation. (3) Artificial selection; man selects for breeding pur- 

 jioses suitable individuals, and prevents a new character which has 

 arisen through variation from disappearing through crossing again 

 with animals of the opposite variational tendencies. 



Factors of Evolution in Nature. — If we compare with the facts 

 of domestication the conditions of animals living in the state of 

 nature, we find again variability and heredity, as efficient forces, 

 inherent in all organisms, though the former is not everywhere of 

 the same intensity. There are many species which vary only 

 slightly or not at all, and therefore have remained unchanged for 

 thousands of years. But contrasted with these conservative species 

 are in every group progressive species, active species, which are 

 in the process of rapid change, and these alone are of importance 

 in causing the ajipearance of new species. Since hereditv is 

 present in all organisms, there is only lacking a factor correspond- 

 ing to artificial selection, and this Darwin discovered in the 

 so-called 'natural selection.' 



Natural Selection : Struggle for Existence.— Natural selection 

 finds its basis in the enormous number of descendants which everv 

 animal produces. There are animals (e.g.. most fishes) whicii 

 produce many thousands of young in the course of their lives; not 

 to mention parasites, whose eggs are numbered by millions. For 

 the development of this animal throng there is no room on the 

 earth; for even if we compute upon the basis of a slowly-multiply- 

 ing animal, like the elephant, and assume tluit all the progeny live 

 and reproduce nornuilly, it would only be a few centuries liefore 



