162 FOOT-NOTES TO EVOLUTION. 
that not in all forms is this power of the reproduction 
of the whole organism so sharply limited to the germ 
cells alone. The familiar propagation of plants by cut- 
tings, the regeneration of complete animals from small 
portions of their somatic tissues in many lower forms, 
and numerous other considerations such as these, show 
clearly that the difference between the powers of somatic 
and germinal cells is but one of degree; that while in 
higher organisms the two seem sharply defined from 
each other, a series of lower forms may be taken which 
will show the intermediate steps in this gradual speciali- 
zation of function. 
In the unicellular organisms we have most interest- 
ing examples of the fundamental facts of reproduction, 
and through an examination of these 
we may gain an insight into the more 
complicated processes of the Metazoa. 
Each of these lowest forms consists of a single cell in 
which are carried out in a generalized way the complex 
physiological functions which, in many celled animals, 
are divided up among cell groups. In reproduction the 
animal simply divides into two, the division of the 
nucleus preceding that of the cytoplasm, and the 
method is usually a more or less modified karyokinetic 
one. This mode of multiplication continues in most 
forms for a certain number of generations, and then the » 
necessity for conjugation—i.e., a temporary or perma- 
nent fusion with another individual—sets in. If this 
conjugation be prevented, the animal 
soon shows increasing signs of degen- 
eration which result in death. This “senescence” of 
the powers of growth and multiplication can only be 
checked by the admixture of new zuclear substances 
from an entirely different individual by conjugation. 
In its simplest terms this process is found in Chzlodon, 
Reproduction in 
Protozoa. 
Conjugation. 
