INFLUENCE OF INTELLIGENCE ON INSTINCT 171 
believe, through natural selection that a sufficiently high 
standard of strength 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 
conception of the matter in the clear light of a working 
hypothesis. 
The amount of what we may term “ modifiability ” by use 
differs a good deal in the several organs and tissues. The 
teeth of carnivora and the antlers of deer may be cited as 
structures in which the conditioning effects of use form a 
relatively unimportant factor. On the other hand, the nervous 
system, with which we are here primarily concerned, is of all 
animal structures that in which what is acquired may attain 
the greatest importance in the successful conduct of life ; the 
nature and the range of behaviour affording an index of the 
amount of modifiability in this respect. 
