RACES 25 



very closely. Human beings may be regarded as polymorphic because 

 the child differs from the adult, or they may be considered polymorphic 

 because the adults differ amongst themselves. It is in the latter sense that 

 the term is commonly employed in connection with trypanosomes. It 

 must be recognized, however, that the trypanosomes which are regarded 

 as being polymorphic may not all be in the same stage of development. 

 There is evidence which points to the fact that the shorter stumpy forms 

 of T. brucei or T. gambiense are the result of growth from the long slender 

 forms which are present in the blood at the same time (Figs. 222 and 225). 



RACES. — Amongst Protozoa, as amongst human beings, there occur 

 different races of one and the same species. The individuals of one race 

 differ from those of another in size, shape, rate of multiplication, and other 

 characters. Each race breeds true to its type to a large extent, so that 

 even after long periods of multiplication the same differences are observed 

 in the resulting progeny. On this account it often becomes a matter of 

 difficulty to decide whether two different forms are merely races of one 

 species or are actually different species. Thus, in the case of Entarnoeha 

 histolytica there appear to be several races which can be distinguished from 

 one another by the average size of the cysts they produce (Fig. 10). 



Many researches have been conducted on the race question in species 

 oi Paramecium, Difflugia, and other Protozoa, especially by Jennings. It 

 has been observed that the characters of any particular race tend to remain 

 constant, so that there is considerable difficulty in understanding how these 

 races arose in the first instance. Evidence has, however, been obtained by 

 Jennings (1916) in the case of Difflugia corona and by Middleton (1915) 

 for Stylony cilia, which proves that after long periods of multiplication 

 definite inheritable variations do occur in the descendants of a single 

 individual, and this quite apart from any sexual process. It therefore 

 seems probable that if the observations were continued for a sufficient length 

 of time, it would be possible to separate from the descendants of a single 

 individual various races which would be as distinct from one another as 

 the naturally occurring races. If this were not so, it would be difficult to 

 understand how evolution could take place at all. 



A practical point which arises from the knowledge which has been 

 acquired regarding races of Protozoa is that the separation of species, 

 on account of comparatively slight variations in size, is a very questionable 

 procedure. The literature dealing with parasitic Protozoa contains 

 numerous instances of the establishment of new species merely because 

 the dimensions differed slightly from those of a form previously described. 



Another type of race peculiarity occurs amongst the Ciliata. It was 

 shown by Dawson (1919) that Oxytricha kymenostoma, which normally has 

 both a macronucleus and a micronucleus, may occasionally have the 



