664 The Outline of Science 



cult in the case of a parasite that requires two hosts, as so many 

 do, and we may conclude that those types which were constitu- 

 tionally capable of prolific reproduction would be the successful 

 survivors. It cannot be doubted that many animals enter the door 

 of parasitism, but fail to leave it open for their progeny. So they 

 die out in their asylum. The astonishing thing is that so many 

 have succeeded. The dog is known to have about forty different 

 kinds of parasites; both man and the pig have more. In omniv- 

 orous types the alimentary parasites are always more numerous 

 than in those with a specialised diet. Some constitutions seem to 

 favour or attract parasites; thus the European oak-trees harbour 

 about a hundred different kinds of gall-flies. 



It is not only the number of different kinds of parasites in 

 one host that amazes us, it is also the numerical strength of one 

 kind. Thus there may be 10,000 individuals of one kind of thread- 

 worm in the intestine of one grouse, the minute larvae being swal- 

 lowed along with the leaves and flowers of the heather. 



Another interesting point is that particular kinds of para- 

 sites are usually restricted to one kind of host, one reason being 

 that they cannot be adapted to a variety of surroundings. When 

 the parasite requires two kinds of hosts for the completion of its 

 life-cycle, the host of the adult is in many cases an animal that 

 habitually eats the host of the larval stage. Thus the dog eats the 

 rabbit, and the bladder-worm of the rabbit develops into a tape- 

 worm of the dog. Similarly, the bladder- worm of the mouse 

 becomes the tapeworm of the cat. Man gets his two commonest 

 tapeworms (Tc&nia saginata and Tcenia solium) by eating im- 

 perfectly cooked flesh of ox and pig respectively, for these are the 

 hosts of the bladder-worm stages. 



The larva of the liver-fluke cannot continue its development 

 in Britain unless it gets into a particular species of freshwater 

 snail, called Lymnceus truncatulus or minutus; if it enters another 

 species it is unsuccessful. This is what we mean by specificity or 

 individuality of life-history. And yet the same liver-fluke larva 



