I20 SECRETS OF EARTH AND SEA 



survival in the hands of the breeder had nothing to do 

 with the toes or their web, but was simply " caused " by 

 these pigeons having feathers of "survival or selection 

 value" in his judgment. Male white cats with blue eyes 

 are deaf If deafness were ever an advantage (a difficult 

 thing to imagine), you would get a species of cat with 

 white hair and blue eyes, and be led to distinguish the 

 species by those characters, not by the real cause of 

 survival, namely, deafness. Not enough is yet known of 

 this curious and very important subject of correlation, 

 but its bearing on the signiiicance of " specific characters " 

 is sufficiently indicated by what I have said. 



An interesting group of species, three of which are to 

 be purchased alive through London fishmongers, are the 

 European crayfishes, not to be confused with the rock- 

 lobster or Langouste (Palinurus), sometimes called " craw- 

 fish " in London, nor with the Dublin prawn (Nephrops). 

 The little river crayfishes are like small lobsters, and 

 were placed by older naturalists in one genus with the 

 lobsters. Now we keep the European species of crayfishes 

 as the genus Astacus, and the common lobster and the 

 American lobster have been put (by H. Milne-Edwards) into 

 a separate genus (Homarus). You can buy in London the 

 " ^crevisses a pattes rouges " of French and German rivers, 

 which is called Astacus fluviatilis, and differs from that of 

 the Thames and other English and European rivers 

 (which you can also buy) called A. pallipes ("pattes 

 blanches " of the French), by the bright orange-red tips 

 of its legs, and by having the side teeth of the horn or 

 beak at the front of the head larger and more distinct. 

 The English crayfish grows to be nearly as large as the 

 " pattes rouges " in the Avon at Salisbury, though it has 

 nearly disappeared about Oxford. You can also sometimes 

 buy in London the big, long-clawed Astacus leptodactylus 

 of East Europe. There are two or three other species, 



