NATURAL SELECTION 
2 5 
individuals to propagate their race. The survival of acci- 
dentally favoured individuals will also occur, but in the long 
run the effect of this factor will be nil, as it will sometimes 
select better, sometimes worse, individuals. The importance 
to an animal or plant of even a slight variation can only be 
appreciated after considerable familiarity with outdoor natural 
history has been attained, and even then only in a few cases. 
The very delicate adjustment of the balance subsisting 
between the various organisms of a district and between 
them and their surroundings may be disturbed even by the 
most apparently trivial causes. We know for instance that 
the weight of the individual seeds in any given species is 
variable ; now heavy seeds give stronger plants, and hence, 
as offspring tend to inherit the parental characters, the 
weight of the seed may tend to increase in each generation, 
until further increase becomes disadvantageous, either di- 
rectly or by producing correlated effects on some other 
structure or function in the plant. 
Many recent authors (cf. works mentioned under Varia- 
tion) deny that continuous variations can be thus permanently 
accumulated, and base evolution upon discontinuous varia- 
tion, but for our purpose it is sufficient that variations can 
be accumulated and in time produce such alteration in 
structure and function that it may be extremely hard to 
trace the phylogeny of an organ or plant (cf. Chap. II.). 
Structural and Functional Characters ; Adap- 
tation ; Ecology. It is evident that every existing plant 
or species must be fairly well suited to its surroundings, 
but not necessarily perfectly so ; if it be very well suited to 
existence amid such environment, it will probably increase 
in numbers, if not, it will probably decrease, and perhaps 
ultimately become extinct. Its characters are, as we have 
seen, derived from pre-existing forms, but a certain number 
of them will have been recently acquired in the evolution 
that has occurred. These will often be characters suiting 
it more, closely to the circumstances under which its life is 
carried on, and if hereditary and of direct use in the life- 
history, may be called adaptive characters, or adaptations 
to the mode of life. As time goes on, if the descendants 
continue to live in similar environments, the adaptive cha- 
racters may become more and more pronounced. At the 
