ACQUIRED FEATURES INHERITED 297 



to disappear, or, at any rate, very few specimens are 

 seen. When blood from the snake is inoculated into 

 a clean rat, then trypanosomes reappear in the rat, 

 but they are not like those originally inoculated. It 

 seems certain that, in such a case, changes in form 

 and virulence of the trypanosome have occurred. 

 What is the explanation of the change ? One of the 

 suggestions put forward is that the rat trypanosome 

 has merely undergone certain changes that have 

 enabled it to become different when returned to its 

 original host. While this is possible, it is probable 

 that other explanations of the change of form and 

 of virulence will be afforded later, when the bio- 

 chemistry of trypanosomiasis in vertebrates and 

 invertebrates has been more fully investigated. 



Among lowly organisms, plasticity and power of 

 adaptation to different conditions are very great, but 

 like higher animals, most of the Protozoa retain 

 many of their ancestral characters. The external 

 form may be slightly modified, or the internal struc- 

 ture may be changed temporarily, but often a short 

 period is sufficient to bring about the restoration of 

 the original form. Some very interesting and in- 

 structive cases are known of new " races " of try- 

 panosomes arising owing to the action of certain 

 drugs on the hosts harbouring them. In these cases 

 the blepharoplast has disappeared. The organisms 

 then show only a large nucleus. They move actively, 

 increase in numbers, and behave in an ordinary 

 manner. But the descendants of the blepharoplast- 

 less trypanosome do not revert to the ancestral type 

 for many generations, and consequently the organisms 



