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, z.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 
