190 LEISURE-TIME STUDIES. 



able development, that, practically, the immense number of 

 eggs produced by these animals are of small account. Of 

 the thousands of eggs developed, the merest fraction attain 

 development, and the presence of a complex life-history in 

 parasites must be regarded as in reality forming a saving 

 clause, as far as man is concerned, when we consider our 

 comparative immunity from their attack. 



Even more extraordinary than the phases of develop- 

 ment which have just been detailed, are those undergone by 

 a special form of tapeworm inhabiting the dog. The egg of 

 this latter parasite gains admittance to the body of the dog- 

 louse, and therein becomes the cystic worm, analogous to 

 that formed within the muscles of the pig in the case of the 

 human tapeworm. The dog, in the process of cleaning his 

 skin, swallows the skin-parasite with its contained but im- 

 mature tapeworm ; and, once introduced to the dog's diges- 

 tive system, the latter form liberates itself from the louse and 

 becomes the mature and adult tapeworm. Anything more ex- 

 traordinary than this peculiar circle of development can hardly 

 be imagined in the life-histories of animals. Nor are the con- 

 ditions which have determined, and which continue the de- 

 velopment, rendered clear to us by the most careful study of 

 the subject. Why it is that the tapeworm should not attain 

 its full development within the pig, rabbit, mouse, or dog-louse, 

 as its first host, we do not know ; nor can it be rendered 

 plain what conditions have so sharply divided the life of 

 these parasites into two periods of such well-marked kind. 



The whole question of parasitism, however, exhibits a 

 striking illustration of the influence of habit and of surround- 

 ing conditions on the life of animals. No one may doubt 

 that the habit of one animal attaching itself to another is an 

 acquired one. The most ardent advocate of the doctrine of 

 special creation would never dream of maintaining that 

 parasites were created as we find them in relationship with 

 their hosts. Even were this argument advanced as a mere 

 matter of unsupported belief, the order and succession of 

 life upon the globe would present facts which would at once 



