LAMARCK'S THEORY OF EVOLUTION 267 



degree, essential to their being, in such a way that at 

 the end of many successive generations these indi- 

 viduals, which originally belonged to another species, 

 are at the end transformed into a new species, distinct 

 from the other. 



" For example, if the seeds of a grass, or of every 

 other plant natural to a humid field, should be trans- 

 planted, by an accident, at first to the slope of a 

 neighboring hill, where the soil, although more ele- 

 vated, would yet be quite cool {frais) so as to allow 

 the plant to live, and then after having lived there, 

 and passed through many generations there, it should 

 gradually reach the poor and almost arid soil of a 

 mountain side — if the plant should thrive and live 

 there and perpetuate itself during a series of gener- 

 ations, it would then be so changed that the botanists 

 who should find it there would describe it as a sepa- 

 rate species. 



" The same thing happens to animals which circum- 

 stances have forced to change their climate, manner 

 of living, and habits ; but for these the influences of 

 the causes which I have just cited need still more 

 time than in the case of plants to produce the nota- 

 ble changes in the individuals, though in the long 

 run, however, they always succeed in bringing them 

 about. 



" The idea of defining under the word species a col- 

 lection of similar individuals which p erpetuat e the 

 same by generation, and which have existed tTius as 

 anciently as nature, imphes the necessity that the 

 individuals of one and the same species cannot mix, 

 in their acts of generation, with the individuals of 

 a different species. Unfortunately observation has 

 proved, and still proves every day, that this consider- 

 ation has no basis; for the hybrids, very common 

 among plants, and the unions which are often ob- 

 served between the individuals of very different 

 species among animals, have made us perceive that 



