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SCIENCE 



[N. S. Vol. XXV. No. 632 



nected. Even thus early it was apparent 

 that parasitic animals were derived from 

 free living forms, that they were in fact 

 but degraded members of the same groups ; 

 in some cases with such little modification 

 in structure that their affinities were recog- 

 nized at a glance, in other cases, however, 

 the excessive modification had carried the 

 parasitic form very far from the ancestral 

 type, and yet the existence of a large num- 

 ber of intermediate stages suggested at 

 once that these changes had been gradual. 



In spite of the fact that these studies 

 were among the very earliest of biological 

 investigations, it appears that the recip- 

 rocal influence has rather generally escaped 

 discussion. It is my desire, accordingly, in 

 so far as may be practicable within the 

 limits of time and space of a single address, 

 to set before you the main facts in connec- 

 tion with this other side of parasitism — the 

 influences which are exerted upon the host 

 animal, the changes wrought in it, and the 

 part they play in the problems of general 

 biology. There is a vast amount of in- 

 formation and detail regarding individual 

 species; and in some cases the relations 

 have been investigated with great care 

 from the standpoint not only of the mor- 

 phologist and of the physiologist, but even 

 also from that of the chemist; but in the 

 main these observations stand as isolated 

 and unrelated facts. I have neither the 

 knowledge nor the ability to bring them all 

 together into a concrete whole, but there 

 are certain general headings under which 

 it is possible without especial effort or ex- 

 planation to group some of the phenomena, 

 while others must wait for better knowl- 

 edge or clearer perception than I possess 

 before they can be included in any gen- 

 eralizations. 



In rough fashion it is the custom to 

 class parasites as harmful and harmless. 

 The harmful forms induce pathological 

 changes, stand in some definite relation to 



disease and become, accordingly, of prime 

 importance to the investigator in medical 

 fields. The other forms, while spoken of 

 as harmless or without effect, certainly 

 should not be classed as exercising none. 

 With the exception of certain striking in- 

 stances, especially among those forms which 

 parasitize man, the effects of parasitism 

 are almost unknown, hence they are largely 

 ignored. In the course of time opinions 

 on this matter have changed radically. As 

 I have said in another paper : 



"In the belief of the medical profession 

 two hundred years ago there was no dis- 

 ease, real or imaginary, which was not due 

 to the presence and effect of some kind of 

 parasite. Each ailment had its particular 

 'worm' in its characteristic location. This 

 was a direct result of the endeavor to re- 

 duce every malady to some definite cause, 

 and from a joining of the unknown sick- 

 ness with the parasites, of which they knew 

 as little. Under the influence of study and 

 of increase in knowledge regarding the 

 parasites, such a theory was seen to be 

 untenable, and the movement in the op- 

 posite direction began— a tendency which 

 may be said by this time to have passed its 

 height. In fact, there has prevailed dur- 

 ing recent years among the medical men 

 of this country an exaggerated idea of the 

 unimportance of human parasites." This 

 must give way to a proper conception of 

 the pathological significance of these organ- 

 isms, based upon careful investigations of 

 their actual influence upon the host. 



Tet there are some parasites of which it 

 may fairly be said that even careful study 

 has failed to show any manner in which 

 they affect the host. Thus Looss records 

 of a distome (Heterophyes) commonly 

 found in the human alimentary canal 

 among Egyptian laborers, that, although 

 present in considerable numbers, most care- 

 ful scrutiny fails to disclose any influence 

 which it exerts upon the host. This is 



