108 ZOOLOGY. 



in which the male and female animals differ are recognition 

 marks whereby the presence of one sex is made known to 

 the other. In some animals in which fertilization normally 

 occurs, the ova may develop in the absence of sperm (parthe- 

 nogenesis}. This may have arisen as an adaptation to tem- 

 porary scarcity of males. This view is in some cases supported 

 by the additional fact that parthenogenetic eggs produce male 

 individuals wholly, or in excess. 



146. 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 rejuvenescence? in the introduction of greater 

 variation? in conservatism? Show how these are important as adapta- 

 tions in the struggle for existence. In what groups is parthenogenesis 

 found? Give details of the facts in several cases. 



147. Reproduction and Care of Young. The very rate 

 of reproduction is an adaptation to the severity of the strug- 

 gle for existence experienced by the animals of a given species. 

 Those forms with few enemies and abundant food usually 

 need to produce 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. Similarly 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 hus- 

 bands 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 condition 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 protec- 

 tion, 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, re- 



