KINDS OF PARENTS 69 



the parent. All other differences are secondary to these 

 and grow out of them. The task of forming and caring 

 for eggs is so different from the task of forming and caring 

 for sperms that the parents, at first differing only in the 

 internal organs that produce gametes, come to differ in 

 many external particulars. This is known as sex- 

 dimorphism. This is the state we are most familiar with 

 in the higher animals and in man. The males and fe- 

 males of most higher animals are more or less 

 conspicuously different from one another. It very often 

 happens that the males of a species of birds, we will say, 

 differ more in general appearance from the females of that 

 species than they differ from males of an entirely different 

 species. Put in another way, if we were to find the male 

 and female of the rose-breasted grosbeak apart in nature 

 without any chance to find out the facts about them 

 except by external appearance, we should undoubtedly 

 think they belonged to different species. These external 

 differences between males and females of a species are 

 found in birds, in many mammals including man, in some 

 fishes and amphibians, and in many insects. 



6. Nature of External DifTerence between Male and 

 Female Parents. It is not possible here to call your 

 attention to all the ways in which the bodies of male and 

 female parents differ. The differences are greater in the 

 higher and more active animals. In general the males 

 are more highly colored, stronger, fiercer, and more 

 active. They often have special organs of combat, and 

 instincts to use these. The female is usually more retiring 

 and less endowed with instinct to seek out her mate or to 

 fight other members of the species. Her structures and 

 instincts relate more to care of the young. You can find 

 many illustrations of these statements, as well as some 

 exceptions to them, by field observation of birds, 

 mammals, frogs, insects, and spiders. 



7. The Causes of the Differences. We are reasonably 



