248 NATURAL HISTORY. 



* In the same climate, the first result of a wide difference 

 of habitat or environment is to give rise to simple 

 variation of the individuals affected by these differences. 

 But in the progress of time the prolonged difference of 

 habitat in individuals which go on living and reproducing 

 themselves under similar conditions, gives rise in them 

 to differences which become to some extent essential to 

 their existence. Hence, in the course of many succeeding 

 generations, individuals which belonged, to begin with, 

 to one species, find themselves ultimately transformed into 

 another, new and distinct species. 



'Suppose, for example, that the seeds of a grass, or of 

 any other plant natural to a humid meadow, should by 

 any chance be suddenly transported to the slope of a 

 neighbouring hill, where the soil, although more elevated, 

 was still sufficiently moist to allow of the plant maintain- 

 ing its existence. Suppose, further, that after having 

 lived there, and propagated itself there for a number of 

 times, it should by degrees reach the dry and almost arid 

 soil of a mountainous ridge. If the plant should succeed 

 in finding a subsistence in its new habitat, and should 

 perpetuate itself during a series of generations, it would 

 ultimately become to such an extent modified, that the 

 botanist who might find it would make out of it a new 

 species. 



1 The same thing happens in the case of animals which 

 have been forced to change their climate, their manner 

 of living, and their habits \ but in their case the causes 

 which I shall subsequently cite demand a still longer 

 time than among plants, before their influence can pro- 

 duce noteworthy changes in individuals.' 



