ORIGINS OF parasitism: evolution of parasites 55 



Hymenoptera, including the most highly developed of all insects, the 

 social bees and ants, are descended from ancestors which were parasitic 

 in their larval stages. Keilin has put forward strong evidence to show 

 that the Cyclorrhapha flies, which include the house-flies, bluebottles and 

 their allies, are likewise descended from ancestors all of which had 

 parasitic larvae, although at the present time only a small proportion 

 have retained this habit. 



It is perhaps obvious that the benefits bestowed on an organism by 

 the parasitic mode of life would be most marked in its immature stages. 

 During the period of maximum growth it is sheltered from the rigours 

 of the outside world and protected from violent changes in the environ- 

 ment. It is also provided with an abundant and constant food supply. 

 Moreover, its close relationship with the host is for a limited period 

 only, which does not involve the permanent sacrifice of independence, 

 nor the loss of those sense organs which constitute its link with life in 

 the outside world. 



The French zoologist, Giard, gave the name of "placental parasite" 

 to the mammalian foetus. Many biologists strongly object to the term 

 "parasite" used in this sense and consider that it cannot be employed to 

 indicate a relationship between individuals of the same species. Never- 

 theless, the fact remains that during foetal life the mammalian young 

 obtains food, water and oxygen from the body of the mother and 

 through the organ of attachment — the placenta — excretes the waste 

 products of metabolism. Various reactions, not always beneficial, set 

 up by the presence of the foetus, are singularly reminiscent of those 

 brought about by an alien organism feeding at the expense of the 

 host. In fact, placental parasitism represents the supremely successful 

 example of this mode of life. It seems possible that a fundamental 

 distinction can be drawn between the parasitic adult and the parasitic 

 young, the full significance of which has not hitherto been fully appreci- 

 ated. In the former, parasitism appears to lead to dependence and a 

 loss of evolutionary potential, whereas in the immature stages, it may, 

 on the contrary, prove to be a successful and progressive step. 



