Maech 29, 1918] 



SCIENCE 



303 



fused with such a mass of fable and super- 

 stition that its value as a guide to a rational 

 life was not large. Moreover these large 

 forms are not the most dangerous parasites, 

 i. e., those of greatest significance for hu- 

 man existence ; and hence in that period 

 the contributions from the study of para- 

 sitology to human welfare were not con- 

 spicuous. 



Something like a century ago students 

 came to see, faintlj'' and slowly at first, that 

 animals affect man very directly because 

 some of them are definite factors in the 

 production or transmission of disease. 

 One can hardly overestimate the importance 

 of the contributions made by zoologists 

 toward the solution of the problems pre- 

 sented in this field. At first the instances 

 of animals as causes of disease appeared 

 rare, and were recognized only among the 

 higher parasites. In particular eases of 

 this type a demonstration was achieved and 

 the evidence became clear as scattered ob- 

 servations of individual workers were 

 brought together by some master mind and 

 formed into a consistent and sufficient ex- 

 planation of the malady. A splendid illus- 

 tration is furnished by the epoch-making 

 investigations of Leuckart and others on 

 trichina. Owen had found in a dissecting 

 room in London a worm encysted in human 

 muscle and had rightly interpreted it as a 

 pematode ; another form was discovered by 

 Leidy in Philadelphia in pig muscle and 

 recognized as identical with that reported 

 by Owen ; and, finally, adult intestinal para- 

 sites were obtained from cases in Germany 

 which had been previously diagnosed as 

 typhoid, and all these were shown to repre- 

 sent only different stages in the life history 

 of a single parasite, the trichina. The nat- 

 ural result of these studies was the formu- 

 lation of a rational prophylaxis which, when 

 adopted, eliminates triehinosLs absolutely 

 from the list of human ailments. 



These early and significant discoveries 

 were accompanied bj' others of lesser im- 

 portance on various other worms but re- 

 mained after all relatively isolated for a 

 long time. The reason is not hard to find. 

 The rapid and brilliant rise of bacteriology 

 to a position of prominence and its over- 

 whelming demonstrations that bacteria 

 were the causes of many diseases led in- 

 vestigators to seek insistently for them in 

 all maladies of an apparently infectious 

 type. Failure to find them was explained 

 ou the basis of defective technic and the 

 possible existence of other disease-produc- 

 ing organisms was generally overlooked. 



To be sure, malaria had been traced to a 

 protozoon, but the instance remained iso- 

 lated for many years until a combination of 

 causes directed scientific attention to the 

 importance of more exact studies on that 

 and allied organisms, and these in turn 

 opened up a new field, that of protozoal dis- 

 eases. In the investigation of this field, 

 zoological students have performed the 

 chief and almost the only extensive work, 

 and the results of their researches have been 

 of tremendous significance to human prog- 

 ress. 



In his "Malaria Studies of a Zoologist" 

 Grassi laid the foundations for the com- 

 plete elucidation of the life history of the 

 Plasmodium malarice and with that the 

 basis for the adoption of communal and in- 

 dividual habits which have freed the world 

 from the menace of a disease that has ex- 

 acted a heavy toll from the great nations 

 of all ages in human history. Coupled with 

 this the demonstration of the role of the 

 mosquito in transmitting yellow fever made 

 possible for instance the building of the 

 Panama Canal and the maintenance of a 

 highly developed and cultured community 

 on the Isthmus, whence less than half a 

 century before the French workers had 

 been driven out by pestilences so frightful 



