December 16, 1904.] 



SCIENCE. 



831 



tion here to the bearing of my thesis upon 

 the recent attempts to utilize parasitism in 

 ridding us of undesirable or noxious ani- 

 mals. In bacteriology there have been at- 

 tempts to destroy field mice and rats with 

 certain species of bacteria. In entomology, 

 parasitism is such a familiar phenomenon 

 that it has been seized upon on a number 

 of occasions to destroy otherwise unassail- 

 able insect pests. 



Leaving out of consideration the pre- 

 sumptive dangers of introducing new 

 species into a locality or country which 

 must always be taken into consideration, 

 although they may be of no significance, 

 we have to consider the chances of success 

 as compared with the cost of introducing 

 and maintaining the parasites. In any 

 event, we need not expect a destruction of 

 the noxious species, for that is not the end 

 of parasitism. A reduction in numbers 

 is all that need be looked for. The new 

 parasite will probably fail to become ac- 

 climated at first, and it may be necessary 

 to reintroduce it for a number of years. 

 During this period some few may become 

 adapted to their environment and con- 

 tinue as parasites. Whether the equilib- 

 rium finally established will be of economic 

 value, must be observed rather than pre- 

 dicted. In bacteriologic experiments of 

 this kind the continued vigorous activity 

 of the bacteria from year to year need 

 hardly be expected. The disease will 

 either die out or continue on a low level 

 of mortality, in accordance ■with the gen- 

 eral laws I have detailed, unless bacteria 

 whose destructive powers are maintained 

 and carefully gauged in the laboratory are 

 distributed at definite intervals. 



In conclusion, I will simply call atten- 

 tion to another problem affecting the fu- 

 ture well-being of mankind, the possibility 

 of new infectious diseases arising in the 

 flux and change incidental to human prog- 

 ress. We have assumed that the capac- 



ity for a parasitic existence probably de- 

 pends on some original offensive power of 

 the microbe which it accidentally possessed, 

 such as toxin production, or the presence 

 of intracellular toxins combined with de- 

 fensive powers. These, possessed inde- 

 pendently of the host, were probably the 

 entering wedges to be further developed 

 or dropped, according to necessity. It is 

 more than probable that all species of bac- 

 teria which possess these rudimentary in- 

 vasive powers have already availed them- 

 selves of the opportunity to become para- 

 sites of animal life, on the one hand, of 

 vegetable life, on the other, and that no 

 startlingly new diseases will arise from 

 saprophytic forms. 



Subsidiary problems there are. however, 

 concerning the modifications and readapta- 

 tions of the parasitic forms already in ex- 

 istence. These may be grouped luider two 

 heads : 



1. The transfer and adaptation of para- 

 sites from one host species to another. 



2. The increase of invasive properties 

 of parasites of the same host. 



Are there any new diseases likely to ap- 

 pear as a result of the successful adapta- 

 tion of parasites of higher animals to the 

 human sub.ject ? This is a legitimate ques- 

 tion, though difficult to discuss, for want 

 of material at present. Among the more 

 important possibilities I will simply men- 

 tion the bovine tubercle bacillus and the 

 hog cholera group of bacteria. The larger 

 number of parasites on animals are so 

 specialized, however, their receptor appa- 

 ratus, according to Ehrlich, may have been 

 so curtailed that parasitism on a relatively 

 distant species may be impossible. 



As regards the second problem, that mi- 

 crobes may gain in invasive power on the 

 same host, the principle I have endeavored 

 to establish would stand in the way of any 

 rise in virulence because the most invasive 

 forms of a varying species would have the 



