52 FLEAS, FLUKES AND CUCKOOS 



highly improbable that in their case every kind of evolutionary change 

 is brought about in the same way. Mites and feather lice, for example, 

 pass generation after generation on the same individual host and, 

 where they are concerned, one can expect to find evolution working in 

 much the same way as it does on the fauna of a small oceanic island. 

 Competition between members of the same species of lice and the same 

 species of mites, both for food and accommodation, must be intense. 

 On the other hand, certain intestinal worms battle in solitude with the 

 host, the elements and space — for them intra-specific competition plays 

 a minor role. 



Some zoologists believe that natural selection, acting upon chance 

 mutations, gradually alters parasites and adapts them to their special 

 mode of life. In experimental breeding of the small fruit fly [Drosophild) 

 several mutations are known to occur producing wingless flies, or flies 

 with sickle-shaped or greatly reduced wings, which are reminiscent of 

 some of the types found in nature in the various species of parasitic 

 louse-fly. Such mutations may be advantageous for a parasite and 

 consequently on certain hosts the wingless type would stand a better 

 chance of surviving and reproducing itself. It is also thought possible 

 that certain features of a parasite's environment, for example, the ecto- 

 parasite's contact with the constant heat of the bird's body, or the fact 

 that many worms and Protozoa are permanently immersed in their 

 food, act in such a way that some types of mutations are favoured or 

 even induced; natural selection would subsequently determine their 

 survival value. This theory may apply especially where minor adapta- 

 tions are concerned — such as the similar comb-like structures found on 

 the bodies of very dissimilar insect ecto-parasites like certain flies, fleas, 

 beetles, lice and bugs. 



On the other hand some biologists argue that in order to start on 

 this peculiar form of existence an animal must be pre-adapted to para- 

 sitism. Baer surmises that the louse-flies possessed a tendency to 

 regression of the wings, blood-sucking habits and viviparity which 

 destined the group to a parasitic life. In support of this theory it must be 

 pointed out that the features which are characteristic of parasites are 

 by no means peculiar to them. Thus, some tapeworms will lay one 

 hundred and fifty million eggs a year, whereas the estimated annual 

 output of a free-living starfish is upwards of two hundred million. 

 Animals such as the limpet and the common goby have efficient 

 suckers with which they cling to wave-swept rocks. Some female deep-sea 



