4i 6 Evolution and Adaptation 



other kind of germ-cells. This occurs, however, only in the 

 lowest forms. A step higher we find that only a part of the 

 cells of the colony are set aside for purposes of reproduction. 

 The cells surrounding these germ-cells may form distinct 

 organs, which may show certain differences according to 

 whether they contain male or female germ-cells. When 

 these two kinds of cells are produced by two separate indi- 

 viduals, the individuals themselves may be different in other 

 parts of the body, as well as in the reproductive organs. 



When this condition is reached, we have individuals that 

 we call males and females, because, although they do not 

 themselves unite to form new individuals, they produce one 

 or the other kind of germ-cell. It is the germ-cells alone 

 that now combine to form the new individual. 



Amongst living groups of animals we find no such complete 

 series of forms as exist in plants, and the transition from the 

 one-celled to the many-celled forms is also more abrupt. On 

 the other hand, we find an astonishing variety of ways in 

 which the reproduction is accomplished, and several ways in 

 which the germ-cells are carried by the sexual individuals. 

 Let us examine some of the more typical conditions under 

 the following headings: (i) sexes separate; (2) sexes united 

 in the same individual; (3) parthenogenetic forms; (4) ex- 

 ceptional methods of propagation. 



1. Sexes Separate ; Unisexual Forms} — Although the ani- 

 mals with which we are more familiar have the sexes sepa- 

 rate, this is far from being universal amongst animals and 

 plants ; and, in fact, can scarcely be said to be even the rule. 

 When the sexes are separate they may be externally alike, 

 and this is especially true for those species that do not unite, 

 but set free their eggs and spermatozoa in the water, as fish, 

 frogs, corals, starfish, jellyfish, and many other forms. In 

 other animals there are sometimes other secondary differ- 



1 Geddes and Thompson's " The Evolution of Sex " has been freely used in the 

 preparation of this part of this chapter. 



