ADAPTATIONS 



337 



function of such markings has never been clearly proved. The 

 songs of birds and the calls of various creatures may serve also 

 as recognition marks. Each species knows and heeds its own 

 characteristic song or cry, and it is a source of mutual pro- 

 tection. The fur-seal pup knows its mother's call, even though 

 ten thousand other mothers are calling on the same rookery. 



In questions of attack and defense, the need of fighting 

 animals of their own kind, as well as animals of other races, 

 must be considered. To struggles 

 of species with those of their own 

 kind, the term rivalry may be ap- 

 plied. Actual warfare is confined 

 mainly to males in the breeding 

 season, especially in polygamous 

 species. Among those in which 

 the male mates with many females, 

 he must struggle with other males 

 for their possession. In all the 

 groups of vertebrates the sexes 

 are about equal in numbers. 

 Among monogamous animals, which 

 mate for the season or for life, 

 there is less occasion for destruc- 

 tive rivalry. 



Among monogamous birds, or 

 those which pair, the male courts 

 the female of his choice by song 

 and by display of his bright 

 feathers. According to the theory 

 of sexual selection, the female con- 

 sents to be chosen by the one which pleases her. It is as- 

 sumed that the handsomest, most vivacious, and most musi- 

 cal males are the ones most successful in such courtship. 

 With polygamous animals there is intense rivalry among the 

 males in the mating season, which in almost all species is in 

 the spring. The strongest males survive and reproduce their 

 strength. The most notable adaptation is seen in the superior 

 sizo of teeth, horns, mane, or spurs. Among the polygamous 

 fur seals and sea lions the male is about four times the size of 

 the female. In the polygamous family of deer, buffalo, and 

 the domestic cattle and sheep, the male is larger and more 



. 



FIG. 197. Egg case of the Cali- 

 fornia barndoor skate, Raja 

 binoculata, cut open to show 

 young inside. Young issues 

 naturally at one end of the 

 skate. 



