312 THE CAUSES WHICH 



organ of scent is^ I conceive, a means of terrestrial migration 

 with certain beetles, and this would likewise accomit for the 

 phenomena of gregarious and processionary caterpillars. 



We have evidence, then, of the existence of constant laws 

 operating to maintain insect species as known to naturalists from 

 external characters of shape and colour, and can show they retain 

 such character as long as the law^s remain in force. But we like- 

 wise witness disturbing causes distributing the species and pro- 

 ducing superficial change ; and as it is evident internal structure 

 is dependent on external form, any variation in the latter must 

 affect the former. Thus when an insect is dwarfed, this can take 

 place, as in worker-bees, only at the expense of the reproductive 

 or other organs ; or when it becomes wingless from climatic 

 causes, the alar muscles by disuse tend to disappear and change 

 the shape of the body. 



Dealing with geological chronology, the phenomena of generic 

 and specific variation should also be applicable in explanation of 

 certain plants and insects of constant character, being discovered 

 confined to various geological soils within the radius of their 

 distribution, or to favourite haunts postulating more than simple 

 dispersion from a centre. And the pale blue of butterflies fre- 

 quenting limestone and chalky downs need evoke no interference 

 in the law of albinism if the honeyed cowslips and downy oxlips, 

 over whose leaves they flutter, are, as reputed by Linnaeus and 

 Professor Henslow, specifically identical with the shadow-seeking 

 primrose, and may be raised from the same root."^ So likewise the 

 local feature of melanism may be regarded as not only manufac- 

 turing annual varieties, but as pervading the black, bro\vn, and 

 drab tribes of the Alpine, Arctic, and woodland faunas, and may 

 give a reason for their dark trait of beauty. 



But in order that a scientifically defined species should become 

 diverse from, or identical with, another so as to cross freely, it is 

 necessary variation should not alone exist as it can be shown to 

 do in shape, including the form of the anal appendages, in colour 

 and organs of sound, but also in odours that allow of recognition 

 and prevent an undue mingling of kind. And when, divesting 

 ourselves of the inaccurate conventionalities of language, we are 



* "British Flora," p. 105. 



