4 ENTOMOLOGY FOR MEDICAL OFFICERS 



then all such individuals are regarded as forming one 

 " species." 



The final test of species, of course, is the power of the individuals 

 composing it to reproduce freely, inter se, fertile offspring of their own 

 kind. This test can rarely be applied, although there are some ento- 

 mologists who appear to think that inferences drawn from the minute 

 pattern of the dried reproductive organs may be used with absolute 

 confidence as a sufficient substitute for it. In the higher animals certain 

 biochemical tests are also, no doubt, final— ^.^., the blood-reaction of the 

 individuals of the species inter se. 



Though the individuals of a species resemble one another 

 very closely, they are not all exactly alike in every minute 

 detail ; more or less they tend to vary, especially in the case 

 of common species with a wide area of distribution. 



If any such variation from the standard of the species 

 have a tendency to recur with any sort of constancy we then 

 have a variety ; and if there is good evidence that any such 

 variety is peculiar to some particular locality, in which it is 

 more or less isolated, it is looked upon as a geographical race 

 or a subspecies. 



What may be supposed to happen in the case of a plastic 

 species in a state of nature is well paralleled — as the immortal 

 Darwin revealed — by what is known to have happened in 

 the case of certain common domesticated species of animals 

 and plants in the hands of man. 



So far we have considered classification from the broad 

 standpoint, as an attempt to follow out pedigree and to 

 reconstruct a genealogical tree. But classification has also 

 to be looked at from the nearer standpoint of the pure 

 systematist, whose main object may be merely to identify 

 species and to tabulate them with extreme precision. In 

 this way we get Classes split into Subclasses and Superorders, 

 Orders into Suborders and Superfamilies, Families into Sub- 

 families, and Genera into Subgenera. 



These divisions are convenient when they emphasise 

 obvious subordinate natural affinities, but they are of little 

 or no value and indeed are often actually misleading when 

 they are based upon the differences seen in some one organ 

 considered by itself Such " single organ classifications " tend 

 to share the defects of a physiological classification a system 



