I 
WHAT ARE SPECIES 
13 
find it to harmonise with the development hypothesis, that 
Darwin devoted the whole of his life to collecting facts and 
making experiments, the record of a portion of which he has 
given us in a series of twelve masterly volumes. 
Proposed Mode of Treatment of the Subject. 
It is evidently of the most vital importance to any theory 
that its foundations should he absolutely secure. It is 
therefore necessary to show, by a wide and comprehensive 
array of facts, that animals and plants do perpetually vary in 
the manner and to the amount requisite ; and that this takes 
place in wild animals as well as in those which are domesti¬ 
cated. It is necessary also to prove that all organisms do 
tend to increase at the great rate alleged, and that this 
increase actually occurs, under favourable conditions. We 
have to prove, further, that variations of all kinds can be 
increased and accumulated by selection ; and that the struggle 
for existence to the extent here indicated actually occurs in 
nature, and leads to the continued preservation of favourable 
variations. 
These matters will be discussed in the four succeeding 
chapters, though in a somewhat different order—the struggle 
for existence and the power of rapid multiplication, 'which is 
its cause, occupying the first place, as comprising those facts 
■which are the most fundamental and those which can be 
perfectly explained without any reference to the less generally 
understood facts of variation. These chapters will be followed 
by a discussion of certain difficulties, and of the vexed question 
of hybridity. Then will come a rather full account of the 
more important of the complex relations of organisms to each 
other and to the earth itself, which are either fully explained 
or greatly elucidated by the theory. The concluding chapter 
will treat of the origin of man and his relations to the lower 
animals. 
