20 THE PROTOZOA 



difficult to detect.* These facts suggest strongly that the parasites 

 produce specific toxins ; but the " sarcocystine " produced by para- 

 sites of the genus Sarcocystis (Sarcosporidia)f is almost the only case 

 up to the present, in which a toxin has been isolated from a Pro- 

 tozoan parasite. Laveran and Pettit (19), however, claim to have 

 obtained " trypano toxins " from trypanosomes. 



Considering the facts of parasitism generally, as a problem of 

 natural history, two guiding principles must be borne in mind 

 clearly : the first is that any organism, parasitic or otherwise, tends 

 to be adapted in the best possible manner to the circumstances of 

 its natural environment ; the second is that, so long as a parasite is 

 entirely dependent on its host for its existence, it is to its utmost 

 disadvantage to bring about the death of its host. When, therefore, 

 a given parasite is constantly lethal to a particular host or hosts, 

 one of two explanations must be sought for : either the case is one 

 of a disharmony that is to say, of imperfectly-adjusted relations 

 between the host and parasite ; or the parasite must obtain from 

 the death of the host advantages in the matter of the continuance 

 of the species sufficient to compensate for the temporary loss 

 through destruction of individuals. 



The conditions to which a parasite requires to be adapted are 

 different in many ways from those that influence the life of a free- 

 living organism. When once a parasite has obtained a footing in 

 its proper host, the problem of food-supply is solved for it, since 

 it finds itself lodged in the midst of abundant nutriment so long as 

 its host lives. On the other hand, if the species is to be main- 

 tained, it is essential that the parasite should be able to infect new 

 hosts, a difficult undertaking, and one in which the chances are 

 all against the parasite in most cases. To insure dissemination of 

 the species a large number of offspring must be produced, and 

 special mechanisms and adaptations may be necessary to insure 

 their reaching their destination. Hence, the more parasites become 

 specialized and adapted to their peculiar mode of life, the more the 

 organs and functions of nutrition tend to become simplified, and 

 the greater the tendency to elaboration and extreme fertility of the 

 reproductive function. 



Considered generally, a parasitic Protozoon reproduces itself 

 within a given host with one of two results : in the first place, with 

 that of overrunning the host and establishing itself there ; in the 

 second place, with that of producing forms destined to infect new 

 hosts. Forms produced in the first manner may be termed the 

 ' multiplicative phases " ; their function is to produce a stock of 

 the parasite. From the stock are given off what may be termed 



* Compare Laveran and Mesnil (391), pp. 146-150. 



f Laveran and Mesnil (18) ; Teichmann (25) ; Teichmann and Braun (26). 



