1906. Moffat. — The Struggle for Existence. 
99 
supposed bird lived eight years or ten years. Living ten 3'ears it 
produces 200 young, of which only two survive, because, on an 
average calculation, there will only be room for two. But if 
it lived only eight years, and produced only 160 young, two 
'.vould still survive ; the difierence would be not in the number 
of survivors, but in the amount of the mortality from compe- 
tition. By increasing either its duration of life or its fertility, 
an animal would insure the sending forth into the world of a 
greater number of individuals doomed to premature death, but 
it would produce only the same number as before of descend- 
ants predestined to a term of successful life. 
It is, therefore, a matter of much importance to form a clear 
conception, one way or the other, of what the struggle for 
existence really means. Does it arise from the earth being 
crowded? Is Darwin correct in his assertion that " each area 
is already fully stocked with inhabitants," so that whenever a 
change of conditions enables one species to increase its 
numbers " other species must decrease" ? If so, it appears to 
me to follow that no species can do itself an atom of good by 
an increase in the average length of life of the individuals 
which compose it. 
On the other hand, if animals are permanentl}' kept below 
their high-water mark b}^ external destructive agencies — if 
for instance, mice are practically saved from having to com- 
pete with one another by the extent to which they are preyed 
on by cats and owls, and if storms and the other perils of a 
long voyage yearly destro}' so man}^ chiff-chafFs and willow- 
wrens on their migration that the survivors need never suffer 
from scarcity of food — we are confronted with what I may call 
an altogether opposite conception of the struggle for life, a 
conception in which catastrophe takes the place that had 
formerl}' been taken by competition. Let us see how this at 
once revolutionises the problem, how can the balance of 
nature be maintained?" On the competition theorj^ the 
answer was perfectly simple. Nature was a self-righting 
machine. Life was kept surging at a certain limit ; beyond 
that limit it could not rise, and below that limit it was much 
too vigorous to fall. Now, we cannot abolish this limit. 
There must be a line, a high-water mark, beyond which animal 
life cannot multiply. But what I call the catastrophe theory 
