SECONDARY OR DIRECTIVE FACTORS. 811 



multicellular organism, reproduced in the usual way, arises 

 from an egg-cell fertilised by a spermatozoon, and the 

 changes involved in and preparatory to this fertilisation, 

 or " amphimixis," may make new permutations and com- 

 binations of living substances or vital qualities not only 

 possible but necessary. 



Secondary or directive factors. i. Natural Selection. 

 The distinctive contribution which Charles Darwin and 

 Alfred Russel Wallace made to aetiology was their theory 

 of Natural Selection. 



By natural selection is meant that process whereby, in 

 the ordinary course of nature, certain organisms, e.g. certain 

 members of the same species, are more or less rapidly 

 and discriminately eliminated, while others are allowed to 

 survive. 



That some forms, e.g. in one family, should succeed 

 less well than others, depends obviously on the fact that 

 all are not born alike, depends, in other words, on the 

 fact of variation. 



That there should be elimination is necessary (a) because 

 a pair of animals usually produce many more than a pair, 

 and the population tends to outrun the means of subsist- 

 ence; and (b} because organisms are at the best only 

 relatively well adapted to their conditions of life, which are 

 variable. These two primary facts and their subsequent 

 consequences, e.g. that some animals feed upon others, 

 that there may be more males than females, etc., render 

 some struggle for existence necessary, though this phrase 

 must be used, as Darwin said, "in a wide and metaphorical 

 sense," including all endeavours for the well-being, not 

 only of the individual, but of its offspring. 



The facts then are that variations constantly occur, that 

 some members of a species or family are necessarily less 

 fitly adapted than others, and that the course of nature is 

 such that these relatively less fit forms will tend to be 

 eliminated, while the relatively more fit will tend to survive. 

 As many variations reappear generation after generation, 

 and may become gradually increased in amount, the con- 

 tinuance of the selective or eliminating process will work 

 towards the origin of new adaptations and new species. 



The importance of natural selection as a secondary 



