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1891. Quantity and Dynamics of Animal Tissues. 977 
immense quantitative differences in animals of different species- 
While the brain of the elephant, the largest of terrestrial mam- 
mals, has the greatest absolute weight, and that of the whale, the 
largest of all mammals, the next in size, yet among the smaller 
animals it is well known that the brain of man is preéminent in 
size. The brains of fishes and reptiles are exceedingly small in 
proportion to the size of the body, and the brain of Coryphodon, 
the prototype of the Ungulata, from the early Tertiary, was rela- 
tively much less than the average existing mammalian brain. 
The several tissues differ immensely in the range of their quanti- 
tative variations. Some of them, as the bones and muscles, grow 
_ under mutual limitations prescribed by the mechanical conditions 
of animal movement. If the animal is to move it must have 
motor tissues, and it must have hard parts upon which the motor 
tissues canact. The existence of the one necessitates the existence 
of the other, and that in certain quantity as well. The quantity 
of the one is reactionary upon the quantity of the other. If one 
is destroyed the other no longer has any use, and eventually 
suffers degeneration. Like relations subsist between certain cor- 
related organs of the viscera. They act and react upon one 
another, and hence their growth and decline must go on together. 
There are other tissues the growth of which is mutually antago- 
nistic. Darwin and Cope have cited the examples of the Artio- 
dactyla where the evolution of the antlers is accompanied by a 
disappearance of some of the teeth. .Facts of like import are not 
wanting in the case of the other tissues. Cope explains them by 
assuming that the animal has a fixed complement of vitality or 
bathmic force, and hence, by the principle of the conservation 
of energy, an expenditure in one form of growth must be com- 
pensated for by an equivalent suppression of some other form of 
growth. Cephalization is really a special case of this general 
principle, where the lines of maximum growth converge in a com- 
mon direction. The acceleration of parts headward is coincident 
with the abortion of parts tailward. Under this view it is easy to 
suppose that the disappearance of the tail in man was accom- 
plished by an enlargement of his cranial capacity, and the vital 
energy that was expended in its movement has now been trans- 


