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1884.] Growth, tts Conditions and Variations. 1097 
probable that the huge size of the great sea elephant arises from 
its being adapted to some less agile and more abundant food than 
that sought by its smaller kindred. With fish the same rule 
applies. The powerfully armed sharks far exceed any others in 
bulk, fish, as a rule, being inefficiently armed and adapted to 
small sized prey. 
If we leave the vertebrates and consider the articulated animals, 
the same rules hold good. The insects are specially adapted to 
a restricted food supply, and their weapons are usually such as 
enable them to obtain food only in small quantities. The suck- 
ing, boring and rasping implements with which they work are of 
no great efficiency, and the largest insects are the strong-flying 
carnivora with their powerful pincers and jaws. The spiders are 
equally small, from the minute quantity of food which their cun- 
ning brings them. By far the largest articulates are the crusta- 
ceans, in whom the pincers have developed into powerful wea- 
pons, and who find in their water home larger, more abundant 
and more easily overcome prey. But in all these cases the size 
of any particular species is largely the result of the efficiency of 
its weapons, the size and quantity of the food to which it is spe- _ 
cially adapted, and the degree of energy or cunning which it 
needs to capture its food and to escape its foes. 
Thus it would appear that the size of every species of animal 
has a natural limit, about which it may fluctuate, but from which 
it cannot widely depart in either direction unless there occurs a 
marked change in the surrounding conditions or in its organiza- 
tion. Its size depends strictly on the efficiency of its food-taking 
Weapons, the abundance of food to which it is specially adapted, 
or the mass of food material which it can obtain, the degree of 
exertion, muscular or nervous, which it needs to obtain this food, 
the degree of exertion which it needs to escape its foes, and the 
vigor of its reproductive energy. The latter, however, isa fluc- 
tuating element, which acts like the governor of a steam engine. 
The former conditions control the average size at any fixed 
period. But if, through a change of conditions, nutrition becomes 
more abundant, reproduction is correspondingly checked and 
size increases, while the reverse occurs if nutrition decreases. 
Thus by a correlation of its two powers of nutrition and repro- 
duction each animal manages to hold its own in the struggle for 
existence, If, through any causes, an animal tribe falls below the 
