OBSERVATIONS CONCERNING FISH-PARASITES. 109 



in a long term of years may determine the relative abundance or rarity of the individ- 

 uals of a species. In most cases the interrelations between parasite and host have 

 become so adjusted that the evil wrought by the parasite on it's host is small. Para- 

 sitic forms, like free forms, are simply developing along the lines of their being, but 

 unlike most free forms they do not contribute a fair share to the food of other crea- 

 tures. I have been too busy with the practical work of describing actual species to 

 speculate on the question of their utility, and, moreover, have had no experience in 

 teleological reasoning. It does appear to me, however, that if the question could be 

 referred to all the rest of animated creation, there would be practical unanimity in 

 the declaration that those animals which pass all or the greater part of their existence 

 as parasites are supremely unnecessary. 



IV. 



I am sometimes asked substantially this question : Of what use- is the study of 

 these comparatively obscure forms? In answer I must admit that a large majority of 

 the species belonging to the orders Trematoda, Cestoda, and Nematoda, which fur- 

 nish about all the entozoa, have little other than a zoological interest. But as I under- 

 stand scientific investigation it very often does not concern itself about questions of 

 practical utility. 



Again, I am asked : What remedy is proposed for those cases of parasitism which 

 are plainly injurious? A few remarks on the subjects suggested by these two ques- 

 tions may not be inappropriate. As to the utility of helminthological investigations, 

 it may be said that they stand in the same category as any other scientific investi- 

 gation. Where there is exhaustive knowledge of the thing itself the application of 

 that knowledge towards getting good out of it or averting evil that may come from 

 it first becomes possible. For example, a knowledge of the life-history of Trichina 

 spiralis and its pathological effects on its host has taught people a simple way of 

 securing immunity from its often deadly effects. A knowledge of the life-histories of 

 the various species of taeniae which infest man and the domestic animals, frequently 

 to their serious hurt, has made it possible to diminish their numbers, and may in time 

 lead to their practical extinction. 



So with the parasites of fishes. Whenever for any reason or reasons parasitism of 

 any sort becomes so prevalent with any species as to amount to a disease, the remedy 

 will be suggested, and in some cases may be practically applied. If, for example, it 

 were thought desirable to counteract the influences which are at work to cause the 

 parasitism of the trout of Yellowstone Lake, it could be very largely accomplished by 

 breaking up the breeding-places of the pelican on the islands of the lake. With re- 

 gard to Darasitism among the marine food-fishes, the remedy, while plainly suggested 

 by the circumstances, might be difficult of application. Yet something could be done 

 even there, if it were thought necessary to lessen the amount of parasitism. If such 

 precautions as the destruction of the parasites which abound in the viscera of fish 

 before throwing them back into the water, and if no opportunity be lost of killing 

 those sharks which feed on the food- fishes, two sources of the prevalence of parasites 

 would be affected and the sum total of parasitism diminished. These remarks are 

 made not so much because such precautions are needed, as to suggest possible appli- 

 cations of knowledge which is already available. 



