306 
Entamoeba histolytica and E. ranarum 
difference may be expressed by saying that E. histolytica and E. ranarum 
are, in all probability, different species. This is the chief conclusion 
which I draw from the foregoing experiments. 
I do not, of course, ignore the fact that biological classification rests 
fundamentally upon morphology—upon structural and not upon 
functional characters. When, therefore, I draw the above conclusion 
from physiological data only—from an observed difference in the 
behaviour of the cysts of the two different “species”—1 do not imply 
that such data are, alone, sufficient to establish a specific difference. 
I take them as an indication that E. histolytica and E. ranarum differ 
from one another in important functional characters; and that the 
difference is so considerable, that when the life-histories of both species 
are fully known, it will then be found that they are likewise distinguish¬ 
able morphologically. There is already direct evidence that this is so: 
for the early development of E. ranarum in the tadpole, as described by 
Collin (1913), appears to be quite different from anything known to 
occur in the life-history of E. histolytica in man. It is, however, still 
premature to discuss these points, since the life-history neither of 
E. histolytica nor of E. ranarum is yet known in complete detail. 
Nor is it justifiable to conclude directly, from the fact that E. histo¬ 
lytica cysts will not give rise to an E. ranarum infection in the frog, that 
a converse infection is equally impossible. It is still possible that the 
cysts of E. ranarum, if swallowed by a man, might infect him with 
E. histolytica, and thus make him suffer from amoebic dysentery. It is 
still possible, though perhaps improbable, that E. histolytica and E. ran¬ 
arum are the same species: but that after the amoeba has established 
itself in man, its cysts have ceased to be infective for the frog. In any 
case, the transference from the cold-blooded to the warm-blooded host 
would have to be accompanied by extensive changes in the habits of 
the parasite. It would have to change from a harmless commensal, 
feeding upon the contents of the intestine, into a pathogenic parasite 
living upon its tissues. At present there is no indication that such a 
change is possible. 
It must be confessed that the hypothesis that E. histolytica is the 
human form of E. ranarum —a natural parasite of amphibia, accidentally 
acquired by man—is not without attractiveness. It would agree very 
well with the distribution of amoebic dysentery and its seasonal incidence. 
If true, it would also suggest certain preventive measures directed 
against the frog. There is, however, as yet no direct evidence to incrim¬ 
inate the frog as a source of amoebic infection in man, and in tropical 
