SECONDARY OR DIRECTIVE FACTORS. 8rr 
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.—1. Matural 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 the same species, are more or less rapidly 
and discriminately eliminated, while others are allowed to 
survive. 
That some forms, ¢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 (4) 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, ¢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 
