This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1908. Excerpt: ... In a paper by Rothschild (1906, p. 4831) it was stated that the species of flea usually found on rats in tropical and subtropical countries is Pulex cheopis Rothsch. When investigating the fleas which occur on any given host it appears to us to be essential to ascertain if there is more than one species associated with the spread of disease in the host, and to demonstrate precisely, in order to avoid confusion, the characters by which the various allied species can be distinguished. It is obvious that false conclusions will be drawn by investigators who maintain that numerous similar (though in fact specifically distinct) forms are identicaI. Such observers have usually examined their specimens with eyes untrained for the special line of investigation upon which they have entered, with the result that they deny the existence of differences which are obvious to the specialist. We shall, therefore, before describing the various species of Loemopsylla with which we are acquainted, point out the reasons which guide us in the discrimination of the differences which we consider to be specific and those which we consider to be non-specific. The criterion of a "species" is a biological one. The fauna of a district consists of a certain number of units, each producing its own kind only; each being independent of the other in that respect; their own organisation only keeping these units apart. Such a unit we call the "species." Each species varies more or less and all individuals composing a species are different from one another. Again a species may be split up into well marked varieties, sometimes with and sometimes without the existence of intergradations. All individual varieties are, nevertheless, specifically identical, standing either in the relation of brot...