54 FLEAS, FLUKES AND CUCKOOS 



instance, the sexual form of certain roundworms develops directly from 

 the egg in sheep, but in rabbits the same worm produces an asexual 

 generation. In a few abnormal hosts worms are dwarfed, or only one 

 sex — the male — -may achieve development. Again, variable strains of 

 trypanosomes are known which are dependent upon and produced as a 

 response to the environment in a particular vertebrate host. 



Whatever theory is favoured, from the point of view of the biologist, 

 parasites remain a particularly interesting and fruitful study. For, 

 although there are no fossils with which to compare parasites, free- 

 living forms from which they must have been derived are often available 

 and the two can be examined alive side by side. The zoologist can look 

 at an active free-swimming copepod dashing about in the water with 

 its antennules twitching and its swimmerets beating, and he can also 

 examine the parasitic fish louse, attached like a small sack of blood and 

 eggs to the host — and stare in amazement at the results of evolution. 



It is probable that parasitic animals exceed non-parasitic forms, 

 both in the number of existing species and in the number of actual 

 individuals. For example, from man — not counting bacteria and fungi 

 — over five hundred different species of parasites are recorded. This 

 mode of life consequently appears, at first sight, to be highly profitable. 

 However, the evolution and progressive transformation in the direction 

 of successful parasitism clearly reduces and circumscribes the possibility 

 of future readjustments. Huxley has defined biological progress in its 

 broadest sense as "control over the environment and independence 

 from it." The evolutionary trend of parasites is in the opposite direc- 

 tion — towards dependence. 



We have already called attention to the fact (p. 7) that many 

 animals can be parasitic for some period of their lives and yet show no 

 trace of this particular mode of existence, either in form or function, 

 during other stages of their life-cycle. There are also those cases in 

 which one sex is parasitic and the other is not, and the free sex displays 

 no modifications which can be attributed to the strikingly different 

 way of life chosen by its mate. Furthermore, whether we are dealing 

 with a coot or a cuckoo, a butterfly or a bed-bug, an earthworm or a 

 lungworm, we find that the eggs and sperm of both free-living and 

 parasitic animals are remarkably alike. 



It would certainly appear that a parasitic existence during the 

 larval stages of an animal's life-cycle is neither so harmful nor so 

 irrevocable as in the adult stages. Many entomologists believe that the 



