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 Paramcecia produced by the conjugat- 

 ing individuals a body which contains part of the body 

 substance of two distinct individuals. The new Paramce- 

 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 



