50 ANIMAL PHYSIOLOGY. | 
Fie. 56.—Head of a nose-ape (Sem- Fie. 57.—Head of Julia Pas- 
nopithecus nasicus) from Bor- trana. (From a photo- 
neo. (After Brehm.) graph by Hintze.) 
constitute fixed differences by which a new group is marked 
off. In the determination of the variations that persist, the law 
of survival of the fittest operates. 
REPRODUCTION. 
As has been already noticed, protoplasm, in whatever form, 
after passing through certain stages in development, undergoes 
a decline, and finally dies and joins the world of unorganized 
matter; so that the permanence of living things demands the 
constant formation of new individuals. Groups of animals 
and plants from time to time become extinct; but the lifetime 
of the species is always long compared with that of the individ- 
ual. Reproduction by division seems to arise from an exigency 
of a nutritive kind, best exemplified in the simpler organisms, 
When the total mass becomes too great to be supported by 
absorption of pabulum from without by the surface of the 
body, division of the organism must take place, or death ensues. 
It appears to be a matter of indifference how this is accom- 
plished, whether by fission, endogenous division, or gemmation, 
so long as separate portions of protoplasm result, capable of 
leading an independent existence. The very undifferentiated 
character of these simple forms prepares us to understand how 
each fragment may go through the same cycle of changes as 
the parent form. In such cases, speaking generally, a million 
individuals tell the same biological story as one; yet these 
must exist as individuals, if at all, and not in one great united 
mass. But in the case of conjugation, which takes place some- 
times in the same groups as also multiply by division in its 
various forms, there is plainly an entirely new aspect of the 
