D.—ZOOLOGY. 119 
The significance of an imported case of plague depends in large measure 
on the local species of Xenopsylla. Hirst has made numerous attempts 
during the plague season in Colombo to transmit plague by means of 
X. astia from rat to rat, but with negative results, and X. aslia was 
never found to behave like a ‘ blocked’ cheopis. 
The distinction of X. cheopis from X. astia is not an entomological 
refinement with purely systematic significance, but corresponds with a 
different relation of the species to the epidemiology of plague, and 
hence becomes a factor of great practical importance. If through these 
researches it has become possible by examination of the rat-fleas of a 
locality to estimate accurately its liability to plague, anti-plague 
measures may henceforward be restricted to those areas in which plague 
is likely to occur, i.e. where cheopis is the predominant flea. Thus a 
great economy of effort and of expenditure and a higher degree of 
efficiency may be achieved; in fact, the problem of the prevention or 
reduction of plague may be brought from unwieldy to practicable pro- 
portions. When it is remembered that since we last met in Liverpool 
some ten and a quarter millions of people have died in India from 
plague we have a more than sufficient index of the importance of a 
precise knowledge of the systematics, structure, and bionomics of the 
insect-carrier of Bacillus pestis. 
Another of the outstanding features of the period under review has 
been the extensive and intensive study of the Protozoa. ‘The structure 
and the bionomics and life-history of these organisms have been inyesti- 
gated with the help of the finest developments of modern technique. 
It is fitting here to record our acknowledgment to two staining methods— 
Heidenkain’s iron-hematoxylin and the Romanowsky stain (including 
Giemsa’s and Leishman’s modifications), which have added greatly to 
our technica] resources. 
There is time to refer only to certain of the Protozoa which directly 
affect man. Twenty years ago our knowledge of the few species of 
Protozoa recorded from the human alimentary canal was defective in 
two important respects—the systematic characters and the biology of 
the species—so there was much confusion. Subsequent investigations, 
and especiaily those of the last ten years (by Wenyon, Dobell, and 
others), have cleared up most of the doubtful points, but owing to the 
difficulties of size and the paucity of characters available it is by no 
means easy in practice to distinguish certain of the species. Of the 
seventeen species now known to occur in the intestine of man 
Entame ba histolytica has received particular attention. This organism 
lives ‘as a tissue parasite in the wall of the large intestine, where, as a 
rule, the damage caused is counterbalanced by the host’s regenerative 
processes. But when the destruction outstrips the regeneration intes- 
tinal disturbance results, leading to the condition known as ameebic 
dysentery. The specific characters and the processes of reproduction 
and encystment of H. histolytica are now well ascertained, and it is 
realised that in the majority of cases the host is healthy, acting as a 
‘carrier’ dangerous to himself, for he may develop into a case 
of acute dysentery, and to the community—for he is passing in his 
