LAMARCK'S THEORY OF DESCENT 283 



He then points out the difficulty of determining 

 what are species in certain large genera, such as 

 Papilio, Ichneumon, etc. How new species arise is 

 shown by observation. 



" A number of facts teaches us that in proportion 

 as the individuals of one of our species are subjected 

 to changes in situation, climate, mode of life or 

 habits, they thereby receive influences which gradu- 

 ally change the consistence and the proportions of 

 their parts, their form, their faculties, even their 

 structure ; so that it follows that all of them after a 

 time participate in the changes to which they have 

 been subjected. 



" In the same climate very different situations and 

 exposures cause simple variations in the individuals 

 occurring there ; but, after the lapse of time, the con- 

 tinual differences of situation of the individuals of 

 which I speak, which live and successively reproduce 

 under the same circumstances, produce differences in 

 them which become, in some degree, essential to their 

 existence, so that at the end of many successive gen- 

 erations these individuals, which originally belonged 

 to another species, became finally transformed into a 

 new species distinct from the other. 



" For example, should the seeds of a grass or of 

 any other plant natural to a moist field be carried by 

 any means at first to the slope of a neighboring hill, 

 where the soil, although more elevated, will yet be 

 sufficiently moist to allow the plant to live there, and 

 if it results, after having lived there and having 

 passed through several generations, that it gradually 

 reaches the dry and almost arid soil of a mountain 

 side ; if the plant succeeds in living there, and per- 

 petuates itself there during a series of generations, it 

 will then be so changed that any botanists who should 

 find it there would make a distinct species of it. 



" The same thing happens in the case of animals 



