1899] THE SCOPE OF NATURAL SELECTION 121 
even if the actual number of inhabitants remain more or less the same 
as when the struggle first commenced. The selection of variations 
will thus tend to pass through certain more or less ill-defined but 
nevertheless real stages. In proportion as the struggle becomes in- 
tense, either from the number or from the increasing adaptability of 
the organisms, or both, certain major essential adaptations, which were 
necessary for the climatic and other more or less comparatively simple 
conditions, will be supplemented by minor auxiliary variations which 
in the earlier stages would not have appeared. And still later as 
more and more rigorous conditions of life were imposed the advantage 
would tend to rest with those organisms which possessed highly co- 
ordinated adaptations, since this would entail more rapid responsiveness 
to environment. 
As evolution advances from the unspecialised to the specialised, 
and higher and higher forms of life come into being, with increasing 
complexity and specialisation of parts entailing an increasingly deli- 
cate adjustment of those parts to each other’s needs, the relation of 
each part to the whole organism becomes of more and more import- 
ance, and it follows that selection must become more and more gener- 
alised in its action. No single variation could be of service to any of 
the higher forms of life unless it was in more or less complete harmony 
with the whole tendency of the individual. The adjustment of parts 
and their mutual interdependence make it essential for adaptation 
that the relation of parts be preserved; consequently, correlated 
minute favourable variations will tend to be more and more selected 
as evolution passes from the unspecialised to the specialised forms of 
life. This response of the whole organism should be still more deli- 
cate in those forms of life that are continually subjecting themselves 
to changed conditions; hence this delicacy of adjustment is far more 
necessary in the higher forms of animal life than in the more stationary 
plant organisms, and in the developing nervous system of animals 
we have just the central adjusting system that is required for these 
conditions. With evolution of type there will thus be an increasingly 
definite tendency given to organic, especially the animal, forms of life, 
if the acting principle of evolution has been selectional. Selection 
is therefore able to account for the steadily progressive tendency ot 
life as a whole without calling to its aid any unknown and doubtful 
perfecting principle. 
To summarise :—Natural selection, acting on the whole organism, 
tends to produce more and more definite tendencies in all surviving 
forms of life, which tendencies are progressive and continuous in 
character. Variable conditions, by partially altering the line of 
selection, induce a temporary indefiniteness. And lastly, the process 
of selection being itself able to be the indirect, though not the direct, 
cause of those favourable variations, which it subsequently selects from, 
is able to dispense with any subsidiary factors, provided it has a 
