142 THE STRUGGLE FOR LIFE. 
and size of any particular feather. The only limitation to 
this power is, that the selection must necessarily be carried 
out in one definite direction, so that it would hardly be 
possible to produce a bird with the powers of flight of a 
Homing Pigeon combined with the crop of a pouter. For 
the sum of the vital forces in any individual is strictly limited 
by the amount of food it can consume and assimilate, and 
therefore excessive growth or superiority in any part is always 
accompanied by dwarfing and inferiority in others. 
I will now take one instance only to illustrate the methods 
by which Natural Selection can modify a species, and for this 
purpose use an animal, now, unfortunately for the picturesque- 
ness of the world, rapidly becoming extinct—I mean the giraffe. 
The ancestor of the giraffe, must have been an animal brow- 
sing on the leaves of shrubs, and with no disproportionately 
long neck. Some, however, would be born with longer, and 
others with shorter necks, just as men are born different in 
the length of their limbs. As they wandered in search of food 
some would reach places where the leaves of the shrubs were 
less easily reached. Those born with rather longer necks 
would then have a slight advantage in procuring food, and 
would be more likely to thrive and leave offspring, and this 
process would be repeated in the next generation, and so on, 
until the giraffe as we see it now was produced. But why 
should the process ever stop ? and why do we not see giraffes 
with necks long enough to reach the loftiest trees of the forest ? 
Natural Selection also determines this. Individuals continue 
to be born with longer or shorter necks, but now increased 
length of neck is a positive disadvantage, because the greater 
weight of bone and muscle which must be carried would be an 
impediment to locomotion and the necessary change of feeding 
ground, while more food is necessary to maintain the nutrition 
of the greater bulk of tissue. 
I only give two examples, those of the pigeon and giraffe, to 
illustrate my meaning, but the study of any other two species 
