228 



THE PHILOSOPHY OF BIOLOGY 



should expect to find that all the variations were 

 equally frequent in their occurrence, but this is not 

 what a study of variability in such a case as we have 

 supposed — that of the animals inhabiting an isolated 

 part of land — does actually indicate. What we should 

 find would be the conditions represented by the diagram 

 B. There would be two or more modes, that is, values 

 of the variable character which are represented by a 

 greater number of individuals than any other value 

 of the variation. The environmental conditions favour 

 the individuals displaying this variation to a greater 

 extent than they favour the rest. 



5" 4- 3 £ 



3^ 



A 



4^3 8 / Q I B i ■^ 3 Z ' a I S 'i '^ 



Fig. 24. 



That is to say, the environment selects some kinds 

 of variations among the many that are exhibited, and 

 this is, of course, the essential feature of the hypothesis 

 of the transmutation of species by means of natural 

 selection of variable characters. Organisms enter the 

 world differently endowed with the power of acting on 

 the medium in which they live, or on the environ- 

 ment consisting of their fellow-organisms. Those that 

 are most favourably endowed live longest and have 

 a more numerous progeny than those that are less 

 favourably endowed, and they transmit this favour- 

 able endowment to their offspring. Among the progeny 

 of the progeny there may be some in which the favour- 

 able variation is still more favourable than it v/as when 

 it first appeared. Thus the variations which are 



