THE MULTIPLICATION OF ANIMALS AND SEX 55 
of its nucleus. Then the two individuals separate and 
each divides into two. The result of this conjugation is 
to give to the new Paramecia produced by the conjugat- 
ing individuals a body which contains part of the body 
substance of two distinct individuals. The new Purame- 
cia are not simply halves of a single parent; they are parts 
of two parents. If the two conjugating individuals differ 
at all—and they always do differ, because no two individual 
animals, although belonging to the same species, are exactly 
alike—the new individual, made up of parts of each of them, 
will differ from both. We shall, as we study further, see 
that Nature seems intent on making every new individual 
differ slightly from the individual which produces it; and 
the method of multiplication or the production of new indi- 
viduals which Nature has adopted to produce the result is 
the method which we have seen exhibited in its simplest 
form among the simplest animals—the method of having 
two individuals take part in the production of a new one. 
The further study of multiplication among animals is the 
study of the development and elaboration of this method. 
32. Differentiation of the reproductive cells—Among the 
colonial Protozoa the first differentiation of the cells or 
members composing the colony is the differentiation into 
two kinds of reproductive cells. Reproduction by simple 
division, without preceding conjugation, can and does take 
place, to a certain extent, among all the colonial Protozoa. 
Indeed, this simple method of multiplication, or some modi- 
fication of it, like budding, persists among many of the com- 
plex animals, as the sponges, the polyps, and even higher 
and more complex forms. But such a method of single- 
parent reproduction can not be used alone by a species for 
many generations, and those animals which possess the 
power of multiplication in this way always exhibit also the 
other more complex kind of multiplication, the method of 
double-parent reproduction. Conjugation takes place be- 
tween different members of a single colony of one of the 
