THE MULTIPLICATION OF ANIMALS AND SEX 57 
38. Sex, or male and female.—In the slightly complex 
animals one individual produces both egg cells and sperm 
cells. But in the Siphonophora, or colonial jelly-fishes, stud- 
ied in the last chapter, certain members of the colony pro- 
duce only sperm cells, and certain other members of the 
colony produce only egg cells. If the Siphonophora be 
considered an individual organism and not a colony com- 
posed of many individuals, then, of course, it is like the 
others of the slightly complex animals in this respect. But 
as soon as we rise higher in the scale of animal life, as soon 
as we study the more complex animals, we find that the 
egg cells and sperm cells are almost always produced by 
different individuals. Those individuals which produce 
egg cells are called female, and those which produce sperm 
cells are called male. There are two sexes. Male and 
female are terms usually applied only to individuals, but 
it is evidently fair to call the egg cells the female reproduc- 
tive cells, and the sperm cells the male reproductive cells. 
A single individual of the simpler kinds of animals pro- 
duces both male and female cells. But such an individual 
can not be said to be either male or female; it is sexless— 
that is, sex is something which appears only after a certain 
degree of structural and physiological differentiation is 
reached. It is true that even among many of the higher 
or complex animals certain species are not represented by 
male and female individuals, any individual of the species 
being able to produce both male and female cells. But this 
is the exception. 
34. The object of sex.—Among almost all the complex 
animals it is necessary that there be a conjugation of male 
and female reproductive cells in order that a new individual 
may be produced. This necessity first appears, we remem- 
ber, among very simple animals. This intermixing of body 
substance from two distinct individuals, and the develop- 
ment therefrom of the new individual, is a phenomenon 
which takes place through the whole scale of animal life. 
