170 INTELLIGENT BEHAVIOUR 
severe exertion, collapse, perhaps death, would ensue if such 
exertion were imperatively demanded under special circum- 
stances. And it is clear that many wild animals must be not 
infrequently placed in such circumstances as will subject their 
muscular structures and the functional activity of their organs 
of circulation and respiration to a strain nearly up to their 
extreme limits of endurance. The carnivorous hunter would 
often fail to secure his prey if his organization were unequal to 
a hard and prolonged chase; the hunted prey would not 
survive to procreate his kind if he fell a victim to the first 
pursuer through inability to stand the exertion necessary to 
enable him to make good his escape. It is thus, we may 
believe, through natural selection that a sufficiently high 
standard of streneth and functional endurance is maintained. 
The failures in these respects are steadily eliminated. It is 
difficult to realize the great strain put upon a bird’s organization 
by the migration flight. Some ten times as many birds leave 
our shores in the autumn as return to them in the following 
spring. What proportion of these is weeded out in the act of 
migration we do not know; but we may be sure that only 
those fitted to stand a severe test of physical endurance return 
to rear broods which shall inherit in large degree similar vigour 
of constitution. 
Two factors, then, determine the limits of efficiency in the 
bodily organs—heredity and use. And these two co-operate in 
such a way that we may say, either that due use is the essential 
condition of the effective development of the hereditary powers, 
or that heredity serves to condition their effective development 
through use. But though closely related, so that each may be 
regarded as conditional on the other, they are, if we accept the 
view that acquired characters are not transmitted as such, so 
far independent in that use adds nothing to, disuse subtracts 
nothing from, the hereditary store. It is, indeed, difficult 
to conceive how, on any view, the absence of the conditioning 
factor of normal use can be the efficient cause of a positive 
diminution of the balance at the bank of heredity. And 
Lamarckian thinkers have not succeeded in placing their 
