Sro THEORY OF EVOLUTION, 



etc.) is complex and variable. Moreover, every multi- 

 cellular 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 combina- 

 tions of living substances or vital qualities not only possible 

 but necessary. 



Secondary or directive factors. \.NaturalSelection.- 

 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 

 eliminated, while others are allowed to survive longer. 



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 subsistence ; 

 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 

 continuance 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 



