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 
