Definition of species. 



That this must be so is shown by the occasional appearance 

 of " monstrosities," which differ markedly from their progenitors, 

 but which cannot be classed as belonging to a different species 

 because they are incapable of handing down to succeeding genera- 

 tions such abnormal differences. By many authorities a third 

 defining limit has been applied to the term " species." They believe 

 that true species, when intercrossed, are often sterile, and, when 

 fertile, the hybrids produced are almost always sterile. Varieties, 

 on the other hand, are almost always fertile. 



The idea of species, then, must rest on three orders of facts : 

 (l) the morphological resemblances between individuals; (2) the 

 lineal transmission of distinctive characters ; and (3) the sterility of 

 first crosses between species or their hybrids. 



Although the practical application of this definition of species 

 is involved in many difficulties, we may reasonably expect that some 

 attempt to conform to it should be made by naturalists. It is well 

 known that entomologists are particularly neglectful in this respect, 

 and that they have recorded a vast number of forms as distinct 

 species on the strength of single specimens and without any know- 

 ledge of their generation. In the large majority of cases the species 

 of mosquitoes have been established solely on the morphological 

 characters of dead specimens. We think it is not unreasonable to 

 expect that before an entomologist decides to class any mosquito as 

 the representative of a new species, or a new genus, he should 

 consider whether it represents merely an " individual difference," 

 a " monstrosity " or a " variety." To workers in India it is well 

 known, for example, that the different individuals hatched out from 

 the same batch of mosquito eggs vary greatly in size, and we have 

 found that the adults developed from any batch of eggs of A. 

 culici/acies can almost always be readily divided into a group of 

 large and a group of small individuals. Yet size is a character not 

 infrequently relied upon by Mr. Theobald for the separation into 

 distinct species of two mosquitoes alike in other respects. His species 

 Myzomyia minutus. for example, which was described from a single 

 specimen forwarded to him from Lahore, would appear to have been 

 founded upon this character alone, and the same character is used as one 

 of the points of distinction between A. funestus and A. rhodesiensis. 

 Under the heading of " individual differences " comes also the question 



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