54 THE AMOEBAE LIVING IN MAN 



somewhat curious, that these authors — who deny that E. histolytica has 

 races producing cysts of small size — have nevertheless included " E. 

 hartmanni Prowazek, 1912" in their list of synonyms of " E. dysenteriae," 

 which is the name they give to E. histolytica (Mathis and Mercier 

 (1916) p. 644); though Prowazek's " E. hartmanni'' was actually a 

 strain of E. histolytica producing small cysts. 



Malins Smith (1918) has recently cast doubts upon the existence of 

 more than two strains of E. histolytica. He admits that there is an 

 "ordinary strain" having cysts with an average diameter of i2"6 /it, and 

 a "small strain" with cysts of 77 /i. He seems, however, to have reached 

 this conclusion as a result of a fallacious method of investigation. He 

 measured small samples of cysts from 30 different infections (not more 

 than 50 from any individual case), and thus obtained measurements of 

 the diameters of 1000 cysts. Plotting these graphically, he obtained a 

 bimodal curve with modes at 7'i /t and 12*2 11. He then says : "There 

 seems to be no doubt from the curve that the cysts of this species 

 divide themselves naturally into two strains, differing only in size, with 

 dimensions as indicated in the curve." But this conclusion is surely 

 unwarranted. If there are strains of E. histolytica which differ in the 

 diameters of their cysts — as the figures which Miss Jepps and I have 

 published do, 1 think, prove conclusively — then Smith's curve does not 

 show how many such strains there are, and what mean diameters they 

 individually possess, but merely the frequency with which strains of 

 different sizes occurred in the sample of 30 infections which he studied. 

 His curve merely shows — on the basis of small samples from each — that 

 strains having the larger-sized cysts were most plentiful in his material, 

 strains with small cysts less numerous, and those with cysts of inter- 

 mediate and very large size absent. His own figures (his Table I, p. 34), 

 so far as they go, seem to me quite consistent with our results : but his 

 conclusion that there is only evidence of the existence of two strains, 

 with average cyst-sizes of 77 fi and 126 /i, can hardly be deduced from 

 his curve. He has not even demonstrated the existence of a single 

 strain with either of these diameters as its mean.* No adequate number 

 of individuals was studied from any one strain ; and how any legitimate 

 conclusions can be drawn by treating the problem in such a manner 

 I do not understand. One might as well attempt to show that there are 

 no differences in stature among the various European races of man by 

 measuring the heights of a few individuals from each race and then 

 striking an average for the whole lot ! It is clearly impossible to 

 show by Smith's method whether racial differences do or do not exist. 



It seems to me, therefore, that there is a fundamental fallacy in the 

 method which Smith has adopted ; and consequently I do not think it 

 necessary to discuss the details of his work further. Nevertheless, I am 

 glad to note that he admits that the species E. histolytica can be divided 

 into at least two different races, distinguishable by the sizes of their 

 cysts ; and I have but little doubt that, if he continues his investigations, 

 he will be able to convince himself of the existence of others. 



His Cases 24, 25, and 26 have an average diameter of I2"6 m, but only 50, 20, and 

 Z7 cysts respectively were measured from each of these. With such small samples 

 the conclusion hardly amounts to a demonstration. I can find no single case recorded 

 in which the cysts had an average diameter of 77 m — a figure which seems to result 

 from taking the mean of at least two distinct strains. 



