of the Piedmontese Coast, 173 



tinue the route. Even at the greatest rate assigned by Sir 

 Charles Lyell to oceanic currents (three miles per hour), it would 

 take thirty or forty days for the fry of a Saxicava to traverse 

 the Atlantic ; and it is not reasonable to suppose that the de- 

 velopment of the animal would be postponed for such an extra- 

 ordinary period, or its vitality suspended, for the purpose of its 

 migration, — to say nothing of the innumerable obstacles that 

 would occur in its passage, from cross currents, being snapped 

 up by other animals for food, or a subsidence into some deeper 

 part of the sea or abyss from which it could not extricate itself. 

 It seems to me more probable that the species in question was 

 at its first creation diffused over the whole of the ocean, and 

 that the area of its habitability was afterwards limited by some 

 accidental circumstance, such as a deposit of mud, which choked 

 and exterminated the animal in the intermediate districts. 

 Something like this I have noticed on the coast of South Wales, 

 in the case of a once extensive colony of Pholas dactylus having 

 become extinct within the memory of living man, in consequence 

 of the bed of peat which they had inhabited having silted up 

 and been covered with sand and mud by the action of the tides. 

 Changes of climate, and many other conditions on which the 

 habitability of such animals depends, may have contributed to 

 confine the original area for other species within narrower limits ; 

 and it is therefore not necessary to resort to the theory of mi- 

 gration, or diffusion of species from one province to another, in 

 order to account for their present distribution. 



Mr. Searles Wood, in his account of a British Crag shell 

 (Pyrula reticulata) j which he considers to be identical with a 

 species now inhabiting the Indian Ocean, is of opinion that 

 certain shell-fish which formerly lived together, but are now 

 found to inhabit different climates, have since retired or mi- 

 grated into those parts of the world, the one north and the 

 other south, where the temperature of both is very different 

 from that which must have been favourable to their existence at 

 the period anterior to the formation of the Coralline Crag, and 

 that they have therefore in some degree changed their nature 

 in assimilating such extremes to their present existence; and he 

 assumes that their dispersion was effected by oceanic currents 

 in opposite directions. He, in fact, attributes the changes 

 which have taken place in geographical distribution, not to any 

 alteration in the temperature, but to an alteration in the habits 

 of the animals themselves, caused by gradual migration. But 

 I cannot help recalling to my mind the apophthegm of the old 

 poet, which appears applicable as well to the nature of inferior 

 animals as to that of mankind : " Coelum, non animum, mutant 

 qui trans mare currunt." I consider it to be far more likely 



