418 Evolution and Adaptation 



3. Parthenogenetic Reproduction. — It has long been known 

 that, in some cases, eggs that are not fertilized will begin 

 to develop and may even produce new individuals. Tichomi- 

 roff showed that by rubbing with a brush the unfertilized 

 eggs of the silkworm moth, a larger percentage would 

 produce caterpillars than if they were not rubbed. During 

 the last few years it has been shown that the development 

 of a non-fertilized egg may be started in a number of ways. 

 Such, for example, as by certain solutions of salt or of 

 sugar, by subjecting the eggs to cold, or by simply shaking 

 them. 



There are certain groups of animals in which the males 

 appear only at regular (in others at irregular) intervals. In 

 their absence the females produce eggs that develop without 

 being fertilized, i.e. parthenogenetically. The following exam- 

 ples will serve to show some of the principal ways in which 

 this " virgin reproduction " takes place. In the group of 

 rotifers the males are generally smaller than the females 

 and are usually also degenerate. In some species, although 

 degenerate males are present, they are unnecessary, since 

 parthenogenesis is the rule. In still other species no males 

 exist and the eggs develop, therefore, without being fertilized. 

 In some of the lower crustaceans parthenogenesis occurs in 

 varying degrees. In Apus males may be entirely absent at 

 times in certain localities, and at other times a few, or even 

 very many, males may appear. Some species of ostracod 

 crustaceans seem to be purely parthenogenetic ; others repro- 

 duce by means of fertilized eggs ; and others by an alterna- 

 tion of the two processes. The crustaceans of the genus 

 Daphnia produce two kinds of eggs. The summer eggs 

 are small, and have a thin shell. These eggs develop with- 

 out being fertilized, but in the autumn both male and female 

 individuals develop from these unfertilized eggs, and the 

 eggs of the female, the so-called winter eggs, are fertilized. 

 These are also larger than the summer eggs, have thicker 



