THE DOCTRINE OF EVOLUTION 103 



next generation, those individuals varying in the direction 

 desired are again chosen for breeding. If the process is con- 

 tinued through several generations it may produce a race in 

 which the special characteristic desired is fixed. Artificial 

 selection is one of the means by which the different races of 

 domesticated animals and plants have been produced. 



According to Darwin, something similar to this artificial 

 selection by man goes on in nature, producing the different 

 species of animals as we know them to-day. All organisms 

 tend to vary. Some variations, he says, are due to changes in 

 the conditions of life and to excess of food (these two under- 

 lying the great variability of domesticated animals) ; others 

 are due to the nature of the organism ; to the inherited 

 effect of habit and the use and disuse of parts ; or to rever- 

 sion to characters once possessed by ancestors. The varia- 

 tions. which occur are of two kinds, definite and indefinite. 

 Indefinite, or fluctuating, variations are those comparatively 

 slight differences which occur constantly among animals and 

 plants, so that of all the individuals of the same species, 

 no two are ever exactly alike. Definite variations are more 

 striking ; of these the an con ram is an example. This ani- 

 mal, born in Massachusetts in 1791, of an ordinary breed of 

 sheep, had a long back and short, crooked legs. From it, 

 by crossing, has been produced the ancon breed of sheep, 

 showing the same peculiarities as its progenitor. The breed 

 was highly prized for a time on account of the inability of 

 the animals to jump fences. Darwin lays most stress upon 

 the fluctuating variation as affording the material for natural 

 selection. * 



In order to understand the principle of natural selection, we 

 must consider for a moment the struggle for existence. This 

 term is used by Darwin in " a large and metaphorical sense, 

 including dependence of one being on another, and including 

 (which is more important) not only the life of the individual 



