SELECTION 151 



correlated with some useful character ; and he has 

 himself pointed out that many of these so-called 

 useless characters are useful as recognition-marks ; 

 that is, marks by which the individuals of a species 

 are enabled to recognise each other. This theory 

 of recognition-marks is probably true with reference 

 to some of the higher animals, but it is very im- 

 probable that either form or colour could be used 

 as recognition-marks among the lower animals, and 

 the theory obviously does not apply to blind animals 

 and plants. Even among animals possessing eyes 

 there are many characters which cannot be regarded 

 as recognition-marks, for they cannot be seen : as, 

 for example, the teeth on the tongues of snails, and 

 the internal convolutions, of the suture in the shells 

 of ammonites. The neuration of the wings of moths 

 and caddis-flies is obscured by scales or hairs, and 

 yet it often furnishes good generic and sometimes 

 specific characters. Some crabs are always covered 

 with seaweeds growing on their shells, and the 

 species to which they belong cannot be ascertained 

 until these seaweeds have been removed. And 

 generally, obscure characters cannot be explained as 

 recognition-marks., when there are conspicuous 

 characters to answer that purpose. These specific 

 characters therefore are not recognition-marks ; are 

 they adaptations of any other kind '? We can hardly 

 suppose that the colours which distinguish the shells 

 of some bivalves which live buried in sand have any 

 adaptive value. Nor can we suppose that a spine 

 more or less, or a different arrangement of the 

 tubercles on the carapace of a crab can give one 



