II 6 ZOOLOGY 



149. Mating Adaptations. — One of the most striking forms 

 of individual variation is seen in the differences between the 

 sexes of higher animals. The male and female are often so 

 widely different in form, size, color, and other qualities, that 

 naturalists have sometimes erroneously classified them as be- 

 longing to different 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 repre- 

 sented in the same individual. In such cases the elements often 

 mature at different times. An individual is thus alternately 

 male and female. This is regarded by many as being the primi- 

 tive condition, — the separation of the sexes being accomplished 

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

 individual. Some think that the temperature and the amount 

 and quality of food have something to do with the proportion 

 of males and females which are produced. So sexual dimor- 

 phism in some species may be in some measure a response to 

 external conditions and presents every evidence of being an 

 advantageous adaptation to the conditions of life. On the other 

 hand, it is believed by many that the sex of most organisms is 

 determined by conditions in the egg and sperm that unite. In 

 other words it is thought that sex is inherited, and cannot be 

 changed by external conditions. 



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 descendants. So too are 

 the wonderful internal mechanisms and tendencies that cause 

 the definite unions and separations of chromosomes in the sex 

 cells as these prepare for fertilization. The chemical attraction 

 which the female cell exerts on the motile sperm cell is a special 

 adaptation which accomplishes this union. Furthermore it is 

 undoubtedly true that many of the color markings, notes, 

 motions, and the like in which the male and female animals 

 differ are recognition marks whereby the presence of one sex is 

 made known to the other. 



Of very first importance as adaptations in reproduction are 



