SECONDARY OR DIRECTIVE FACTORS. 769 



But why should there be changes in the germ cells ? 

 Perhaps, because living matter is very complex and un- 

 stable, and because it is of its very nature to differentiate 

 and integrate ; perhaps because the immediate environment 

 of the germ cells (blood, body cavity fluid, sea water, &c.) 

 is complex and variable. But it may be more important 

 to recognise that every multicellular organism, reproduced 

 in the usual way, arises from an egg cell fertilised by a 

 spermatozoon, and that the changes involved in and pre- 

 paratory to this fertilisation, or "amphimixis," make new 

 permutations and combinations 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 

 etiology 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 a 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 sub- 

 sistence, 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, &c., 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 



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