LAMARCK’S THEORY OF EVOLUTION 267 
degrec, 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. 
“Mor example, 1f the seeds of 2 erassyor 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 (/razs) 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 sfeczes a col- 
lection of similar individuals which perpetuate the 
same by generation, and which have existed thus as 
anciently as nature, implies 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 
