Checks on Increase 
the increase of a species and of the comparative 
powerlessness of the attacks of raptorial creatures. 
The failure of the sandgrouse to establish a 
footing in England is, we believe, due to the fact 
that it is constitutionally unfitted to withstand our 
damp climate. 
The camel is an animal that revels in dry 
habitats, hence the difficulty of keeping camels 
in damp Bengal, although they seem to thrive 
well enough in the drier parts of India. 
‘When a species,” writes Darwin (p. 86), 
“owing to highly favourable circumstances, in- 
creases inordinately in numbers in a small tract, 
epidemics—at least, this seems generally to occur 
with our game animals—often ensue; and here 
we have a limiting check independent of the 
struggle for life. But even some of these so- 
called epidemics appear to be due to parasitic 
worms, which have from some cause, possibly 
in part through facility of diffusion amongst 
the crowded animals, been disproportionately 
favoured: and here comes in a sort of struggle 
between the parasite and its prey.” 
Thus inadequately does Darwin deal with that 
bar to the increase of organisms, which is only 
second in importance to the effect of climate. 
The check occasioned by disease and parasites 
is one to which naturalists have as yet paid but 
little attention. The result is a very general 
misunderstanding of the true nature of the 
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