Serological criteria of species

Abstract

HERE is no need to remind a medical audience that treatment can be considered only after diagnosis. Should the disease be caused by some pathogenic micro-organism it is essential to determine accurately the species involved. The kernel of many problems in zoology, including a number of problems in medical zoology, has been, and indeed still is, the recognition of the species involved. Applied biology can be a science only if based on sound systematics. Sometimes the recognition of valid species needs little effort. Only the G.P.I. would confuse elephant and rat, but on the other hand, even the most sober and erudite might have difficulty in distinguishing the common rat fleas, or even fail to realise there was more than one species of rat flea. When it became clear that plague was a zoonosis, transmissable by fleas from the natural rat reservoirs, the Plague Commission was puzzled by the fact that outbreaks of plague never lasted long in Colombo and Madras, despite the abundance of rats and rat fleas. The problem was solved by Lord Rothschild who found that the flea normally occurring on rats in these cities was Xenopsylla astia Roths., a species similar to but separate from the plague flea X. cheopis Roths., and unlike it, a very poor vector of plague. A calculation of the number of individuals of the common rat flea of this country which one would have to examine: to find two specimens with identical chaetotaxy produces a figure of several billions, far more than the total population of these fleas in Ireland (Rothschild, 1932). Where between this extreme and that of a genus does one draw the line that indicates a species ? This brings us at once to the question of what is a species. This is truly a $64,000 question: the answer is already known, in practice if not in theory, by any person one is likely to want to question about the matter. What is this answer ? "A species is what a competent sys