40 FLEAS, FLUKES AND CUCKOOS 



among animals which Hve in the sea than on land. There is one famous 

 example of a male roundworm which lives as a parasite inside the 

 vagina of the female. There is no doubt that this is a certain way of 

 ensuring that sperms are available when the eggs are ready for fertilisa- 

 tion, but like so many other devices to which parasites resort, it is 

 rather an exaggerated form of the more usual relationship between the 

 sexes. Among some ticks which also attack birds there are cases of 

 dwarf males parasitising the females to which they become permanently 

 attached (see tail-piece, Chapter 12). They pierce the skin of their 

 mates and gorge themselves on blood recently extracted from the body 

 of the host. Many internal parasites, however, have solved the problem 

 of fertilising their eggs in another way. Both male and female organs 

 are found in the same individual. Such two-sexed animals are known 

 as hermaphrodites and in many cases they are capable of self-ferti- 

 lisation and are completely independent. Marital worries are unknown 

 as far as tapeworms are concerned, for they can produce millions of 

 offspring in complete peace and solitude. Some hermaphrodites — for 

 instance quite a large proportion of trematodes — do, nevertheless, 

 copulate with another individual if the two should happen to meet in 

 the heaving darkness of the bird's intestines. There is no question of 

 waiting for the right sex — as copulation can take place between any 

 two mature individuals, a mutual penetration by the male organs 

 occurs and cross fertilisation results. Each partner then lays eggs. 



Various parasites, for instance some ticks and nematodes, have 

 found yet another solution to the same problem. They resort to virgin 

 birth and in such cases their eggs develop without being fertilised. 

 Sometimes this form of procreation, which is known as parthenogenesis^ 

 goes on for several generations, but when a male happens to be available 

 the female returns once again to the more usual form of reproduction. 

 In some species, however, only females have been found and it is believed 

 that the male sex has been dispensed with altogether. 



In this way parasites are forced to adopt a dangerous procedure, 

 for asexual reproduction reduces the variability of the species con- 

 cerned. New combinations of mutations by sexual cross-fertilising 

 cannot occur, and such characters will remain isolated in each asexually 

 produced line or population. Huxley has stressed that the sexual 

 process confers a greater plasticity in evolution, and the parasite is 

 forced to sacrifice evolutionary potentialities by adopting partheno- 

 genesis, polyembryony and strobilisation in its efforts to reproduce 



