SEXUAL DIMOBPHISM 



INTRODUCTION 



In investigating the history, and attempting to discover the 

 causes, of the evolution of the animal kingdom, it is of great 

 importance that we should have a correct conception, and 

 a logical analysis of the phenomena to be explained, the 

 phenomena of animal structure. In the first place structural 

 differences divide animals into groups which are separated in 

 different degrees in relation to reproduction and descent. 

 This is the natural result of descent with modification. The 

 degree of difference between separate groups will depend on 

 the time during which modification in different directions 

 has taken place, and the rate at which the changes have 

 been produced. The ultimate groups are those which we call 

 species, or in some cases the smaller subdivisions which we 

 call varieties. The individuals of a species are united in the 

 great majority of cases not merely by a great degree of resem- 

 blance to one another in structure, but by the fact that they 

 consist of males and females which unite together in the pro- 

 cess of sexual reproduction, while the individuals of one species 

 do not interbreed as a rule with those of others. Groups of 

 species are united together by resemblances of less degree 

 into genera, genera into families, families into classes and 

 orders, and finally the classes into the great sub-kingdoms. 

 The first class of structural peculiarities, then, are those which 



