DIFFERENTIATION OF INDIVIDUALS AND ADAPTATION I17 



the sex attractions and the accompanying emotional states and 

 expressions which are seen in birds, higher mammals, and man. 

 In man these are at the basis of the home and family and the 

 higher personal and social life which arises from the family. 



Ova of some species may develop in the absence of sperm. 

 This is known as parthenogenesis, and is regarded as having 

 arisen from the more common bi-sexual condition. In some 

 species it is exceptional, in others the predominant method, and 

 in still others in which males are unknown it is the only form of 

 sexual reproduction. 



150. Practical Exercises. — What is the difiference in the notes of the male and 

 female of the American quail which would serve as recognition marks? Mention 

 other cases of sexual dimorphism which appear to you to serve a similar end. 

 What evidences have we that the mingling of sperm and ovum results in a re- 

 juvenescence? in the introduction of variation? Show how these are important 

 as adaptations in the struggle for existence. In what groups is parthenogenesis 

 found? Give details of the facts in several cases. 



151. Reproduction and Care of Young. — The very rate of 

 reproduction is an adaptation to the severity of the struggle for 

 existence experienced by the animals of a given species. Those 

 forms with few enemies and abundant food usually need to pro- 

 duce only a few young in order to maintain their place. Others 

 less favored in these regards, as many insects, the lobster, the 

 salmon, must reproduce thousands of young in a lifetime. Simi- 

 larly the length of the reproductive period and of life becomes 

 an adaptation to the same end. It is clear from these facts 

 that any device which the parent may adopt likely to bring a 

 larger percentage of the young to maturity will make for a 

 saving in the necessary birthrate. This husbands the parental 

 resources and conduces to the efficiency of the individual and 

 of the species. It must not be supposed that parental care is 

 confined to the higher animals. In its most elementary condi- 

 tion it takes the form of food stored in the egg, and in depositiiig 

 the egg in a safe place for hatching. After hatching it takes the 

 form of supplying food, or protection, or both. Cephalopods, 

 fishes, and birds have a large amount of food substance stored in 

 the egg. Many animals, as the clam, some fishes, some reptiles, 

 and the mammals, retain the eggs in special portions of the 



