4 Outline of Genetics 



rapidly than the means of subsistence. This increase 

 tends to set up a severe struggle for existence or compe- 

 tition, as the result of which an equilibrium of species 

 is established, with approximately the same number of 

 individuals of a given species surviving year after year 

 in any given locality. Darwin next points out the 

 universality of variation among living organisms, such 

 that no two individuals of any species are ever absolutely 

 identical. As for the cause of variation, no explanation 

 is provided, but the nature of variation is rather clearly 

 outlined. Those variations which are important in the 

 evolutionary process are characterized as quantitative, 

 continuous, and fluctuating. By quantitative it is 

 meant that the variations are differences in the degree 

 of development of some part or feature of the organism. 

 When it is said that the variations are continuous, the 

 implication is that further variations will take place in the 

 same direction as the variations that have taken place 

 in the preceding generations. The term ''fluctuating" 

 indicates that reverse variations will take place as 

 freely as do the progressive variations. According to 

 Darwin, variation of this type is going on in all organ- 

 isms. Since this is true, and since a severe struggle 

 for existence is taking place, it is impossible to escape 

 the conclusion that it is the ''fittest" that survive. If 

 a given species is represented in a certain locality by a 

 thousand young individuals, no two of which are abso- 

 lutely alike, and if there is only enough room or only 

 enough subsistence for one hundred of them ever to 

 reach maturity, it must be true that, in general, it will 

 be those that are the best adapted to cope with the 

 conditions of the environment that are the ones to sur- 



