980 The American Naturalist. [November, 
motor tissues and organs by which to escape those enemies. In 
some cases both lines of development have been pursued in some 
part by the same animal. The locomotive function must certainly 
have played a highly important part in the life of fishes from the 
very first, and in birds its manifestations are even more intense. 
But with the evolution of locomotive powers there arose a need 
of a system of external or internal leverage, and the result of that 
necessity is the exoskeleton and endoskeleton of the Arthropoda 
and Vertebrata respectively. A nutritive apparatus necessarily 
coéxisted with the lime-secreting activity. But when the factor 
of locomotion became added to the animal life new differentia- 
tions of structure were called into being. Function and tissue . 
reacted again and again in endless combinations, and the present 
diversities of organic structure were the result. That result was 
marked not only by qualitative but quantitative peculiarities, and 
it is these peculiarities that we have been discussing. Such, in 
brief, is the historical outline. 
We now pass on to consider the dynamics of the subject. It 
is a patent fact that a division of labor is beneficial to all the 
laboring elements. It is no less true that it results in some 
' degree of dependence between those elements. The degree of 
the dependence is heightened the farther the specialization is car- 
ried. This is eminently true of the animal organism. ‘In its 
simplest expressions as Protameba an absolute independence 
between the parts everywhere prevails. All parts alike respond 
to all stimuli, and no part is anything in particular, but every- 
thing in general. In higher forms the structures are more diver- 
sified, and they assert their individuality by differential responses 
to external stimuli. Moreover, each member gives a complete 
response only when it sustains a connection with all its fellow- 
members. The several members form a joint society, whose 
union is strength and whose dissolution is ruin. Such is the 
polity of the animal body. Now we have seen that the several 
tissues vary in quantity. They vary in the same individual at 
different periods of life, and in individuals of equal age in the 
same and in different species. Such variationsin the tissues must — 
poheslly afet ee e a Suppose we 


