PARASITES 185 



parasitism is very widely spread, and there is probably no animal 

 which does not unwillingly entertain unwelcome guests that make 

 no return for services rendered. As De Morgan sings (in The 

 Budget of Paradoxes] : 



"Great fleas have little fleas upon their backs to bite 'em, 

 And little fleas have lesser fleas, and so ad infinitum\ 

 And the great fleas themselves, in turn, have greater fleas to go on, 

 Whilst these again have greater still, and greater still, and so on". 



The general progress of evolution has been from the less to 

 the more specialized as a result of adaptation to increasingly 

 complex surroundings, but to this parasites constitute a striking 

 exception. Free quarters and free rations having been provided 

 for them, they have taken but little part in the active struggle for 

 existence, and well illustrate the principle of Degeneration. They 

 are on the down-grade, adapting themselves to comparatively 

 simple conditions. Hence we find that complex organs of diges- 

 tion, circulation, respiration, and locomotion, together with nervous 

 system and sense-organs, have undergone more or less reduction 

 in thoroughgoing parasites, though, on the other hand, they have 

 frequently developed special piercing, sucking, and adhesive struc- 

 tures, enabling them to exploit their living food-supply, and to 

 maintain their position. The great danger attending this par- 

 ticular mode of life is constituted by the smallness of the chance 

 of transfer from one host to another. In the more helpless forms 

 this difficulty is often met by the practice of living in two or more 

 different hosts which eat or prey upon one another; the adult 

 egg -producing stage, being the most important, is commonly 

 associated with the strongest and most highly organized of these, 

 the so-called "final host". The biological relations between the 

 successive living refuges is always such as to maintain most surely 

 " the vicious circle of parasitism ". Even more important is the 

 immense fecundity of parasites, a necessary provision, for the 

 chances of survival are extremely small. Leuckart calculated, 

 for example, that any one egg of a tape-worm has only one chance 

 in some 83,000,000 of giving rise to an adult. 



What is called Brood Parasitism, where an animal shirks .the 

 duty of bringing up its own young, will be considered in this 

 section, although it is by no means the same thing as true 

 parasitism. 



