ADAl^ATIONS 



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 nnitual ])ro- 

 tection. The fur-seal pup knows its motlier's call, even liioiigh 

 ten tliousand other mothers are calling on I lie same rookery. 



In questions of attack and defense, tlie 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, tlie term rivalr}' may be ap- 

 plied. Actual warfare is confined 

 mainly to males in the breeding 

 ser.son, especially in ])olygamous 

 species. Among those in which 

 the male mates with many females, 

 he must struggle with otlier 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 jileases her. It is as- 

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

 cal males are the ones most successful in such courtship. 

 With polygamous animals there is intense rivalry among th.e 

 males in the mating season, which in almost all sjKH'ies is in 

 the spring. The strongest males survive and reproduce their 

 strength. Tlie most notable athiptation is seen in the superior 

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

 fur seals and sea lions the male is abo\it four times tlie size of 

 the female. In the i)olyganious family of deer. bulTal(\. anil 

 the domestic cattle aud shcep^ the male is larger and more 



Fig. 197. — Egg case of the Cali- 

 fornia barndoor skate, liaja 

 hinoculata. cut open to show 

 young inside. Young issues 

 naturally at cue end of the 

 skate. 



