August IS, 1905.] 



SCIENCE. 



199 



garding merely even the larger, more con- 

 spicuous parasites of man the wildest ideas 

 were current as to their origin and their 

 effect on the system. Thus tapeworms 

 were supposed to originate from thickened 

 mucus, or from an abnormal condition of 

 the alimentary canal ; and various parasites 

 were from time to time regarded as the 

 causes of cholera, typhoid and other similar 

 diseases. Such views as these prevailed 

 generally even less than a century ago, and 

 it is not strange if, in consequence of more 

 accurate knowledge on these points and of 

 the rejection of such wild theories of dis- 

 ease, the pendulum has swung to the oppo- 

 site extreme and animal parasites have 

 come to be considered of insignificant im- 

 portance in the etiology of disease. 



Two factors tended to strengthen this 

 view and further belittle the possible role 

 of animals as disease producers. In the 

 ,first place, with the possible exception of 

 malaria, no animal organism was known to 

 be the cause of any general disease; and 

 while the animal nature of the Plasmodium 

 malaricB was never doubted in any consid- 

 erable circle, the case stood so evidently 

 isolated that it emphasized all the more its 

 own peculiarity. But even more powerful 

 than this was the rise of a new science, 

 bacteriology. Certain minute plant germs 

 had been found to be the cause of decay, 

 why not of disease? In response to the 

 needs of the case there arose a new tech- 

 nique for handling and studying these 

 forms, a rigorously analyzed series of con- 

 ditions for determining their possible rela- 

 tion to disease ; and a new field of science 

 was organized. Discoveries followed one 

 another with marvelous rapidity and every 

 year saw the elucidation of the cause of 

 new maladies. It seemed as if the secrets 

 of disease had been laid bare ; men had 

 traced the causes to bacteria in many cases 

 with such success that they continued to 



follow the same line in other yet unex- 

 plained diseases, confident that there was 

 only some minor defect in technic which 

 would soon be overcome and the solution 

 obtained. Indeed, the very name disease 

 germs was regarded as equivalent to bac- 

 teria. There is no doubt that success in 

 this direction served to draw attention 

 away from the signs which presented them- 

 selves in other fields and particularly to 

 minimize the animal organism as a causal 

 factor in disease. Recent discoveries of 

 great import which have crowded hard 

 upon each other are disclosing here a new 

 field and stimulating the investigation of 

 neglected territory. Let us now examine 

 seriatim the different groups of animals to 

 secure a clear idea of the role played by 

 each in the production of disease. 



The disease-producing organism works 

 slowly, insidiously, saps the vigor of the 

 infected individual without consuming the 

 substance so as to destroy life by immediate _ 

 destruction of the body. It is clearly not 

 carnivorous, but rather parasitic in habit; 

 consequently among the vertebrates as well 

 as among the largest and most powerful 

 invertebrates, one could not expect to find 

 such forms. These largest species might 

 be carriers of disease or even breeders of 

 sickness, but they could not constitute the 

 immediate cause of the malady. It may 

 be interesting to note in passing an appar- 

 ent exception to this rule. The lamprey 

 eel attaches itself to other fish and is di- 

 rectly the cause of the ulcers on the skin 

 which mark the points of the lamprey's 

 fixation, and of the anemia which follows 

 its blood-sucking and often induces the 

 death of its host. But this instance stands 

 alone. 



In the great majority of cases the disease 

 producers are small organisms or at least 

 gain their entrance into the body of the 

 host in a form so minute as to defv detec- 



