DIFFERENTIATION OF INDIVIDUALS AND ADAPTATION 115 



supported by the additional fact that parthenogenetic eggs 

 produce male individuals wholly, or in excess. 



150. Practical Exercises. What is the difference 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 greater variation? Show how these are 

 important as adaptations in the struggle for existence. In what groups is partheno- 

 genesis 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 depositing 

 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 

 body until development has well begun. The flies lay their 

 eggs in the decaying matter which the young use as food. The 

 solitary wasps seal theirs up in nests with the food (dead or 

 wounded spiders or insects) on which they are to develop. 

 Other insects bore into the tissues of living plants and deposit 

 their eggs, about which "galls" or masses of abnormal vegetable 

 tissue are developed. The ichneumon fly deposits its eggs in 

 the body of some other animal. Thus we see an immense num- 



