254 Darwin, and after Darwin. 
in question consists in supposing that if any particular 
organs in a species are habitually used for performing 
any particular action, they must undergo a structural 
improvement which would more and more adapt them 
to the performance of that action; for in each gene- 
ration constant use would better and better adapt the 
structures to the discharge of their functions, and they 
would then be bequeathed to the next generation in 
this their improved form by heredity. So that, for in- 
stance, if there had been a thousand generations of 
blacksmiths, we might expect the sons of the last of 
them to inherit unusually strong arms, even if these 
young men had themselves taken to some other trade 
not requiring any special use of their arms. Similarly, 
if there had been a thousand generations of men 
who used their arms but slightly, we should expect 
their descendants to show but a puny development of 
the upper extremities. Now let us apply all this to the 
animal kingdom in general. The giraffe, for instance, 
is a ruminant whose entire frame has been adapted to 
support an enormously long neck, which is of use to 
the animal in reaching the foliage of trees. The an- 
cestors of the giraffe, having had ordinary necks, were 
supposed by Lamarck to have gradually increased the 
length of them, through many successive generations, 
by constantly stretching to reach high foliage ; and he 
further supposed that, when the neck became so long 
as to require for its support special changes in the 
general form of the animal as a whole, these special 
changes would have brought about the dwindling of 
other parts from which so much activity was no longer 
required—the general result being that the whole or- 
ganization of the animal became more and more 
