140 ANIMAL LIFE AND HUMAN PEOGEESS 



stages of this development and the exact conditions under 

 which it takes place. It has been found that parasites 

 cannot proceed in their predestined course, and necessarily 

 die, unless certain very special circumstances prevail. 



In some species these factors in success are matters of 

 temperature, humidity etc. In other species the young worm 

 must obtain, within a brief period, shelter and nutriment in 

 some particular invertebrate or even vertebrate host, in which 

 to undergo its essential metamorphosis. Where this is the 

 case the natmahst has been able to indicate means whereby 

 the particular essential intermediary may be eradicated and 

 thus the life-cycle of the parasite be inevitably broken. In 

 fact, it is not too much to say that the elimination of this 

 special class of diseases will eventually depend upon a scientific 

 apphcation of zoological data derived from the study of the 

 migrations, outside the human body, of the parasites con- 

 cerned. 



I now proceed to summarise various discoveries that have 

 been made during recent years regarding the migrations of the 

 more important parasites of man, and to show how in each case 

 the facts obtained have already proved of inestimable value. 



One of the earhest and most important observations was 

 that made by Kuchenmeister on the natuie of the bladder- 

 worms which are found in organs of the body unconnected by 

 natural passages with the outside. These parasites never 

 develop genitalia nor give rise to eggs, and it was difficult to 

 explain how, otherwise than by " spontaneous generation," 

 they came to be present in the tissues or how they could 

 reproduce. For a time it was thought that they were merely 

 tapeworms which had strayed from their normal habitat in the 

 intestine and had become degenerate and dropsical in conse- 

 quence of their unusual environment. This hypothesis was 

 apparently strengthened by the discovery that at one spot 

 in each cyst there was always a bud bearing four suckers, 

 which greatly resembled the head of a normal tapeworm. In 

 1851 it occurred to Kuchenmeister to test the matter by 



