GERMINAL SELECTION. 19 



butterflies the fore wings have the appearance of 

 bark, of old wood, or of a leaf, whilst the hind wings, 

 which are covered while resting, alone are brilliantly 

 colored. On this theory we also understand the excep- 

 tions to these rules. We comprehend why Danaids, 

 Heliconids, Euploids, and Acracids, in fact all diurnal 

 butterflies, offensive to the taste and smell, are mostly 

 brightly marked and equally so on both surfaces, 

 whilst all species not thus exempt from persecution 

 have the protective coloring on the under surface 

 and are frequently quite differently colored there 

 from what they are on the upper. 



In any event, the supposed formative laws are not 

 obligatory. Dispensations from them can be issued 

 and are issued whenever utility requires it. Indeed, 

 so far may these transgressions of the law extend, 

 that in the very midst of the diurnal butterflies is 

 found a genus, the South American Ageronia, which, 

 like the nocturnal butterfly, shows on the entire upper 

 surface of both wings a pronounced bark-coloration, 

 and concerning which we also know (and in this re- 

 spect it is an isolated genus and differs from almost 

 all other diurnal butterflies), that it spreads out its 

 wings when at rest like the nocturnal butterfly, and 

 does not close them above it as its relatives do. There- 

 fore, entirely apart from cases of mimicry, which after 

 all constitute the strongest proof, the facts here cited 

 are alone sufficient to remove all doubt that not inner 

 necessities or so-called formative laws have painted the 

 surface of the butterflies' wings, but that the condi- 

 tions of life have wielded the brush. 



This becomes more apparent on considering the 

 details. I have remarked that the usually striking 

 colorations of exempt butterflies, as of the Heliconids, 



