Some varieties of feathering maintained by pigeon breeders lead to the growth of abundant feathers on the legs (as in Cochin-China fowls), and it is found that these feather-legged pigeons always have the two outer toes connected by a web of skin. If it were a stabilized wild form we should separate it as a species on account of its webbed toes, yet the real selection and 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 significance 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 "crawfish" 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 "écrevisses à 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, named and distinguished, which do not come into the London market.