D.— ZOOLOGY 91 



illustration. When the Commission investigating bubonic plague in 

 India had become definitely convinced that the plague was a rat disease 

 transmitted to human beings through the agency of a particular species 

 of rat-flea, no satisfactory explanation could be found why in Colombo 

 and the city of Madras an outbreak of plague did not last long, although 

 rats and rat-fleas abounded. The puzzle was solved when Dr. Hirst 

 took the matter up and sent to my brother the flea material collected in 

 the towns mentioned during a period when there was no plague and again 

 when an outbreak occurred. The examination of the material proved 

 that the flea ordinarily infesting rats at Colombo and at Madras was not 

 (as the Commission had assumed) the plague-flea Xenopsylla cheopis, 

 but X. asfia, a very similar, but different species, which, by experiments, 

 Dr. Hirst proved to be an inefficient carrier of the disease. Outbreaks 

 of plague occurred in those cities only when grain infested with the plague- 

 carrying A', cheopis was brought from the Punjab. For some reason or 

 other the environment at Colombo and Madras does not suit A', cheopis ; 

 it dies out, and consequently the plague disappears. When during the 

 campaign in Mesopotamia camps became infested with rats, the British 

 Museum could give the reassuring answer to an inquiry that there was no 

 danger of a serious outbreak of plague, because the rat-fleas collected were 

 X. astia, none belonging to X. cheopis. This close connection between 

 applied biology and systematics is well understood by the scientists who 

 are at the head of applied biology, but as yet not by every young scientist 

 full of ardour and pride of knowledge, inclined to rely on his own identi- 

 fications and therefore apt to go astray. The help which the systematist 

 can extend to applied biology, however, is for him only a side-issue or a 

 by-product ; he is a student of pure science, devoting his time to the 

 discovery of new species, of new connections between them and of new 

 facts bearing on the relation between the species and its surroundings, 

 the driving force in this pursuit of knowledge being the irresistible 

 attraction which the subject has for him. 



The describing of new species and finding the right place for them in a 

 given scheme of classification and the identifying of species may seem 

 work of an elementary kind, necessary and useful, but nevertheless rather 

 superficial. If systematics ended there, they might satisfy the collector 

 perhaps, but hardly the scientific mind. But this preliminary work is 

 only a part of systematics, differing from the deeper study of the species 

 as the cataloguing of literature does from the critical study of the contents 

 of literature. A natural classification is based on blood-relationship, 

 and therefore entails an inquiry into the evolution of the species classified. 

 Systematics change from a static study of form into a dynamic study of 

 evolution. For that purpose it is not sufficient to know some character- 

 istic by which species A can be distinguished from species B, or which 

 places A and B into the same genus or into different genera. A species 

 is like a book, which must be read critically and in its entirety. Unfortu- 

 nately the systematist is much handicapped, as in the case of mammals, 

 birds, insects and some other classes he has to be content with the portions 

 of the animal which it is customary to preserve in collections. But even 

 so, the contemplation of the skins and skulls of mammals, of the skins of 



