INDIVIDUAL DIFFERENTIATION 'AND ADAPTATION. IO7 



144. The relations of animals of the same species to one 

 another is an interesting mixture of competition and coopera- 

 tion. In the higher forms the parents instinctively make great 

 personal sacrifices that the offspring may be cared for; the 

 offspring on the other hand struggle with each other for this 

 parental provision. In the classification offered (141) it 

 should be remembered that both friendly and competitive habits 

 and structures are always represented in the same individual. 



145. Mating Adaptations. One of the most striking 

 forms of individual variation is seen in the differences be- 

 tween the sexes of higher animals. The male and female are 

 often so widely different in form, size, color, and other quali- 

 ties, that naturalists have classified them as belonging to dif- 

 ferent species and yet it is very manifest that, though different, 

 the sexes are closely adapted to each other. In the lower types 

 of animals the sexes are frequently represented in the same 

 individual. In such cases the sexual elements often mature at 

 different times. An individual is thus alternately male and fe- 

 male. This is regarded by many as being the primitive condi- 

 tion, the separation of the sexes being accomplished by the 

 repression, so to speak, of one or the other sex in each individual. 

 It is now known that the temperature and the amount and qual- 

 ity of food have something to do with the proportion of males 

 and females which are produced. So sexual dimorphism is 

 in some measure a response to external conditions and presents 

 every evidence of being an advantageous adaptation to the 

 conditions of life. The very union of the sperm and the ovum, 

 whereby two cells lose their individuality in one, with a renewal 

 of powers and the mingling of the qualities of two parents, 

 must be looked on as an adaptation of the very highest moment 

 to the animals in which it first appeared, and to their de- 

 scendants. The chemical attraction which the female cell 

 exerts on the motile sperm cell is a special adaptation to 

 accomplish this union. Furthermore it is undoubtedly true 

 that many of the color-markings, notes, motions, and the like 



