September 8, 1916] 



SCIENCE 



345 



velopment, but do not cause symptoms in 

 this animal. The malarial parasite and the 

 yellow fever virus do not seem to injure the 

 mosquito. Nor does the virus of Rocky 

 Mountain fever injure the tick or that of 

 typhus fever the louse. There is, as we 

 know, some degree of parallelism between 

 biological relationship and susceptibility to 

 a given virus. But this susceptibility has 

 nothing to do necessarily with the ability 

 of an animal to serve as an intermediate 

 host or to harbor a parasite. 



The question of adaptation in this con- 

 nection should be mentioned. It is a prob- 

 lem which for a long time has interested 

 the bacteriologist, but by none has its im- 

 portance been more clearly grasped than by 

 Pasteur who was influenced so decidedly in 

 his experimental work on animals by this 

 principle. In the relation of disease to 

 animals most of the important points center 

 around this fundamental idea in one form 

 or another. It goes hand in hand with the 

 principle of specificity. A given organism 

 supposedly specific for a given animal may 

 acquire the property by adaptation of 

 growing in the body of another animal. It 

 has widened its sphere of activity in certain 

 respects but in other respects it is as spe- 

 cific as ever. Specificity like immunity is 

 a phase of the principle of adaptation. In 

 the study of human-animal disease we note 

 that some organisms naturally are adapted 

 to grow in a variety of animals, others lim- 

 ited very decidedly to a particular animal 

 and even to a particular race of a given 

 species. By experiment these latter may be 

 made to widen their sphere of activity very 

 appreciably. As specific illustrations I 

 may cite the early contributions of Welch 

 and the recent work of Gay and Claypole 

 in causing the typhoid carrier state in rab- 

 bits; also the work of Culver in increasing 

 by repeated transfers the resistance of the 

 gonococcus to rabbit serum and the infec- 

 tion of rabbits with such a strain. Some 



strains of bacteria identical when tested by 

 our most refined laboratory methods may 

 be highly pathogenic for a given animal 

 but non-pathogenic for even a closely re- 

 lated species. 



As to the importance of such processes of 

 adaptation in nature for the dissemination 

 of disease from one variety to another or 

 from animal to man, it is very difficult to 

 obtain, experimentally or otherwise, def- 

 inite data extending over a sufficiently long 

 period of time to be of value. "We can at 

 present perhaps conceive of no better hy- 

 pothesis for the origin of infections and 

 their continuance. Bacteria are very old, 

 there being definite evidence, as shown by 

 B. Regnault and by Moodie of the existence 

 of bacilli and cocci in the intestinal canal 

 of animals (coprolytes), in decomposing 

 plant and animal remains and probably 

 also as disease producers in the Mesozoic 

 and Pleistocene era ten to twelve millions 

 of years ago. Far less change were neces- 

 sary in the bacteria to develop into the 

 types of to-day than have occurred in ani- 

 mals since that time. 



There are many infectious diseases whose 

 modes of transmission are at present ob- 

 scure which no doubt will be found to be 

 carried by some lower animal form. As an 

 example of such may be mentioned Rocky 

 Mountain tick fever, in which suggestive 

 evidence exists of a relation to some wild 

 animal, possibly the gopher (spermophilis), 

 as a tick-carrier. The work of Strong and 

 his associates on the two South American 

 diseases, verruga peruviana and oroya, 

 would indicate that they are transmitted 

 by some arthropod. This has not yet been 

 demonstrated. The modes of transmission 

 of many trematode and nematode infec- 

 tions have not yet been discovered, but from 

 what is known of such diseases there can 

 be little or no doubt that many are 

 transmitted through another animal. In- 

 fantile paralysis has been transmitted ex- 



