220 



EVOLUTION AND ANIMAL LIFE 



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 onl}^ to individ- 

 uals, but it is evidently fair to call the egg cells the female 

 reproductive cells, and the sperm cells the male reproductive 

 cells. A single individual of the simpler kinds of animals 

 produces both male and female cells. But such an individual 

 cannot be said to be either male or female, it is sexless — 



that is, sex is some- 

 thing which appears 

 only after a certain 

 degTee of structural 

 and phj^siological 

 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 in- 

 diiaduals, any indi- 

 vidual of the species 

 being al^le to pro- 

 duce both male and 

 female cells. But 

 this is the exception. 

 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 neces- 

 sity first appears, we -remember, among very simple animals. 

 This intermixing of bod}^ substance from two distinct indiAdduals 

 and the development therefrom of the new individual is a 

 phenomenon which takes place through the whole scale of 

 animal life. The object of this intermixing seems to be the 

 production of variation; at least it Avould seem that variation 

 must result from such a mode of generation. By having the 

 beginnings of an organism's body, the single cell from which 

 this whole body develops, composed of parts of two different 

 individuals, a difference between the offspring and the par- 



FiG. 126. — Conjugation of Noctiluca, a one-celled ani- 

 mal: A, Two individuals just fusing; B, the same 

 with cytoplasm wholly fused and nuclei lying closely 

 together; C, the two nuclei in closer fusion; D, the be- 

 ginning of fission. (After Ischikawa.) 



