39^ THE EVOLUTION OF MAN. 



Not until a later period, by the law of sexual selection, so 

 brilliantly elucidated by Darwin, were developed the so- 

 called " secondary sexual characters," that is, those dif- 

 ferences in the male and female sexes which are exhibited, 

 not in the sexual organs themselves, but in other parts of 

 the body (for example, the beard of the man, the breast of 

 the woman) .^^ 



The third important fact, taught us by the lower Plant 

 Animals, refers to the earliest origin of the two kinds of 

 sexual cells. For, as in Gastrseads, and in many Sponges and 

 Hydroids, in which we meet with the simplest rudiments 

 of sexual differentiation, the whole body consists throughout 

 life only of the two primary germ-layers, the two kinds of 

 sexual cells can, therefore, only have originated from cells 

 of the two primary germ-layers. This simple discovery is 

 of extreme importance, because the question of the first 

 origin of the egg-cells as well as of the sperm-cells in the 

 higher animals — and especially in Vertebrates — presents 

 unusual difficulties. In these animals it usually appears 

 as if the sexual cells developed, not from one of the two 

 primary, but from one of the four secondary germ-layers. 

 If, as most authors assume, they do originate from the 

 middle-layer, or mesoderm, the fact is due to an ontogenetic 

 heterotopism, to a displacement in position. (Cf vol. i. p. 13.) 

 Unless the unjustifiable and paradoxical assumption, that 

 the sexual cells are of entirely different origin in the higher 

 and in the lower animals, is accepted, we are compelled to 

 derive them originally (phylogenetically), in the former as in 

 the latter, from one of the two primary germ-layers. It must 

 then be assumed that these cells of the skin-layer or of 

 the intestinal layer, which must be regarded as the earliest 



