l6o THE EVOLUTION OF MAN. 



definite size proper to it, a separation into two new 

 individuals necessarily takes place. Starting from this 

 simplest form of reproduction, we can learn to understand 

 the many complex forms with which we meet, especially in 

 the lower animals and plants. Division is first followed by 

 propagation by buds, then that by the formation of germ- 

 buds, and propagation by germ-cells, or spores. All these 

 forms of multiplication are classed under the name of 

 asexual reproduction, or Monogeny ; in these cases it does 

 not require the union of different individuals to effect the 

 production of new, independent individuals. 45 



The conditions of sexual reproduction, or Amphigony, 

 are quite different. Its nature consists in this ; that two 

 distinct cells must unite in a particular way and blend in 

 order to cause the production of a new individual. As we 

 shall soon return to the subject of sexual reproduction, 

 when we consider the fertilization of the egg, we need not 

 here linger over it. We must only emphasize the fact, that 

 this process of sexual reproduction, in spite of its peculiarity, 

 is yet nearly related to the higher forms of asexual repro- 

 duction, and especially to that by the formation of germ- 

 cells. But while in the latter case a single cell separates 

 from the confederacy of the many-celled organism and 

 forms the foundation of a new individual, — in the former, 

 two different elementary individuals, a female egg-cell and 

 a male sperm -cell, must unite and blend into a single body 

 to effect that purpose. The double cell formed in this way 

 is alone capable of forming by division an aggregate of cells, 

 from which a new many-celled organism then develops. 48 

 (Cf. Chap. XXV.) 



Immediately connected with reproduction is a fifth 



