28 THE PROTOZOA 



tion of new species of parasites, the first step would be the acquisi- 

 tion by the parasite of the power of living in hosts other than that 

 to which it is specific. How such a variation might arise in Nature 

 is impossible to conjecture in the present state of knowledge ; but 

 some experiments that have been carried out upon T. lewisi show 

 that conditions can modify the apparent fixity of its characters. 

 Roudsky (22, 23) found that after prolonged culture on artificial 

 media, and subsequent rapid passages through rats, it was possible 

 to infect mice with T. lewisi. Wendelstadt and Fellmer (27) have 

 shown that T. lewisi, if inoculated into cold-blooded vertebrates, can 

 persist there for a time, and then becomes virulent to rats.* In 

 both cases it is evident that the normal specific properties of the 

 parasite have been induced to vary by changes in the conditions of 

 life, with the result that they become similar to those characteristic 

 of the pathogenic trypanosomes. 



If it be true that a parasite attacking a new host is at first patho- 

 genic to it, but tends in the course of evolution to establish more 

 harmonic relations with the host, the question arises as to how 

 such relations are brought about. There are two organisms con- 

 cerned, and the problem affects them both. In the case of the 

 host the adaptation to the effects of the parasite may be both 

 individual and racial, in the latter case to be perhaps largely ex- 

 plained by the elimination of individuals less fitted by their con- 

 stitution to resist the parasite. In the case of the parasite, also, the 

 problem may be considered from both points of view ; deadly strains 

 of the parasite contribute to their own destruction. Interesting 

 observations bearing on the individual adaptability of strains of 

 Schizotrypanum cruzi have been made by Chagas (425). This para- 

 site, when inoculated into guinea-pigs, was found to kill them in 

 about six days ; this is its initial virulence to this host. After 

 repeated passages through guinea-pigs, it was found that the viru- 

 lence diminished, until guinea-pigs inoculated with strains of attenu- 

 ated virulence lived as much as six weeks before they succumbed 

 to the effects of the parasite. If, when this result had been attained, 

 the parasite was given a single passage through a marmoset, it was 

 then found to have regained its primary virulence to guinea-pigs. 



The study of the exact mechanism of the physiological relations 

 between parasites and their hosts is the task of the investigations 

 upon immunity and kindred problems which now engross so large 

 a share of the attention of scientific workers, but which cannot be 

 considered here in detail. 



Bibliography. For references, see p. 476. 

 * See also Sleeping Sickness Bulletin, No. 22, p. 412, and No. 24, p. 81, 



