254 DARWINISM AND THE PROBLEMS OF LIFE 



the number of parents. They serve to secure the 

 maintenance of the species during the unfavourable 

 conditions. 



The daphnidae are not the only animals whose eggs 

 develop without being fertilised by the male. Quite a 

 number of other species are associated with them in 

 this respect. Many species of small mussels increase 

 parthenogenetically as a rule ; amongst many of them 

 no males have yet been discovered, though they have 

 been carefully watched for years in an aquarium. 

 Parthenogenetic reproduction is also found in the 

 branchiopods and some other species of Crustacea, the 

 gall-flies whose sting causes the gall-nuts on our trees 

 and shrubs, and many lice, including the dreaded 

 phylloxera. They are especially animals that enjoy 

 from time to time very favourable conditions, which 

 they can make use of for most prolific reproduction, 

 and then pass into very dangerous and unfavourable 

 conditions, which they survive in the shape of fertilised 

 and hard-shelled eggs. Hence in this case fertilisation 

 is not directed merely to the multiplication of the species. 

 The best instance of this is the phylloxera. 



In the spring the egg of the phylloxera produces a 

 female which multiplies parthenogenetically to an immense 

 extent, as the animals have an unlimited supply of food 

 in the shape of the vine-tendrils. All these females are 

 wingless, but after several generations the eggs produce 

 winged females which fly from stock to stock and spread 

 the species. They lay two kinds of eggs. The larger 

 produce females and the smaller males. Both are very 

 small and wingless, and cannot feed themselves. After 



