March 29, 1918] 



SCIENCE 



305 



eration the field is vastlj' extended. Both 

 in numbers and variety and probably also 

 in virulence of attack they surpass the 

 metazoan parasites. The volume of the 

 literature dealing with such organisms has 

 increased with great rapidity and new or- 

 ganisms as well as new problems are being 

 discovered every day. When one adds to 

 all this the important problems that concern 

 parasites affecting domestic animals and 

 thus the material welfare of the human 

 race, the extent of the field of work on 

 animal parasites can be to some extent ap- 

 preciated. The mere listing of the names 

 of those who have worked in this field fills 

 pages in every annual bibliography of zool- 

 ogy. 



Such examples as have been cited could 

 be multiplied many fold. Some of the 

 parasites are acknowledged to be serious 

 foes of human progress and if in other 

 cases the effects are less conspicuous be- 

 cause the organisms are confined within 

 narrow geographical limits, still the signifi- 

 cance of their control is no less important 

 to the human race. Furthermore, with the 

 development of commerce and the increas- 

 ing movement of individuals from place to 

 place, disease has lost its earlier local limi- 

 tation and has become an international 

 menace. To all students of social evolution 

 disease looms high a.s a controlling factor 

 in human affairs. History furnishes many 

 records of epidemics that have depopulated 

 great areas, routed victorious armies, and 

 reduced virile and fruitful civilizations to 

 an inefficient and sordid level. Through- 

 out the expanse of the tropics, where nature 

 i-ewards the efforts of the cultivator of the 

 gi'ound in a degree beyond all comparison 

 with the results obtained in temperate cli- 

 mates, human social organization has gen- 

 erally remained on a low plane, or if at any 

 point it took a brief spurt, the end of the 



advance oame quickly and the nation sank 

 to the common level again. 



JIan 's failure thus far to achieve the eon- 

 quest of the tropics may be traced distinctly 

 to the ravages of tropical diseases unres- 

 trained by natural limitations that are 

 found in other climatic environments. 

 Now these diseases are primaril.v those pro- 

 duced by animal parasites and in the con- 

 trol of such diseases lies consequentlj^ the 

 possibility of utilizing the natural resources 

 of the globe at the point of their greatest 

 richness. What greater service could be 

 rendered to humanity than the continued 

 maintenance of highlj- organized social com- 

 munities in the richest regions on the sur- 

 face of the earth? The result already 

 achieved in the Panama Canal zone and at 

 many other individual points in the tropics 

 can be duplicated generally. When this is 

 done the possibilities of human existence 

 will have been enormously enriched. Cer- 

 tainly the demonstration of such a possibil- 

 it.v wliich has already been given in work 

 quoted above should rank as the greatest 

 achievement of zoological studies and in- 

 ferior only to the realization of that possi- 

 bility when the conquest of the tropics is 

 completed because of the mastery of its dis- 

 eases. 



Before closing the discussion, I should 

 like to call j-our attention to another phase 

 which is much less conspicuous than those 

 which have been treated before, but which 

 in my opinion has distinct importance. 

 This is the stimulus given by parasitology 

 to the scientific discussions of biology and 

 consequently to the extension of knowledge 

 and to the general advantage accruing 

 therefrom. Important side lights have been 

 thrown upon morphologj' by study of the 

 processes of modification and degeneration 

 which are so conspicuously exemplified bv 

 parasitic forms. The life history and rela- 

 tionship of these types show many features 



