INFLUENCE OF ENVIRONMENT 111 



the influences of their environment and of the various habits which 

 had been forced upon them in each country, they underwent remark- 

 able alterations and formed various special races. Now man travels 

 about to very great distances, either for trade or any other purpose ; 

 and thus brings into thickly populated places, such as a great capital, 

 various races of dogs formed in very distant countries. The crossing 

 of these races by reproduction then gave rise in turn to all those that 

 we now know. 



The following fact proves in the case of plants how the change of 

 some important factor leads to alteration in the parts of these living 

 bodies. 



So long as Ranunculus aquatilis is submerged in the water, all its 

 leaves are finely divided into minute segments ; but when the stem 

 of this plant reaches the surface of the water, the leaves which develop 

 in the air are large, round and simply lobed. If several feet of the same 

 plant succeed in growing in a soil that is merely damp without any 

 immersion, their stems are then short, and none of their leaves are 

 broken up into minute divisions, so that we get Ranunculus hederaceus, 

 which botanists regard as a separate species. 



There is no doubt that in the case of animals, extensive alterations 

 in their customary environment produce corresponding alterations 

 in their parts ; but here the transformations take place much more 

 slowly than in the case of plants ; and for us therefore they are less 

 perceptible and their cause less readily identified. 



As to the conditions which have so much power in modifying the 

 organs of living bodies, the most potent doubtless consist in the 

 diversity of the places where they live, but there are many others 

 as well which exercise considerable influence in producing the effects 

 in question. 



It is known that localities differ as to their character and quality, 

 by reason of their position, construction and chmate : as is readily 

 perceived on passing through various localities distinguished by 

 special qualities ; this is one cause of variation for animals and plants 

 living in these various places. But what is not known so well and 

 indeed what is not generally beheved, is that every locality itself 

 changes in time as to exposure, cHmate, character and quahty, although 

 with such extreme slowness, according to our notions, that we ascribe 

 to it complete stability. 



Now in both cases these altered locahties involve a corresponding 

 alteration in the environment of the living bodies that dwell there, 

 and this again brings a new influence to bear on these same bodies. 



Hence it follows that if there are extremes in these alterations, 

 there are also finer differences : that is to say, intermediate stages 



