114 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 classified them as belonging 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 represented in the same individual. In 

 such cases the elements often mature at different times. An 

 individual is thus alternately male and female. This is re- 

 garded by many as being the primitive condition, the separa- 

 tion 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 dimorphism in some species may be 

 in some measure a response to external conditions and presents 

 every evidence of being an advantageous adaptation to the con- 

 ditions 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 won- 

 derful internal tendencies that cause the definite unions and 

 separations of chromosomes in the sex cells. 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 in which the male and female animals 

 differ are recognition marks whereby the presence of one sex is 

 made known to the other. In some animals in which sexual 

 fertilization normally occurs, the ova may develop in the absence 

 of sperm (parthenogenesis) . This may have arisen as an adapta- 

 tion to temporary scarcity of males. This view is in some cases 



