ccording as the organ on which they are founded is an essential one or
otherwise.

"With animals, those analogies are most important which exist between
organs most necessary for the conservation of their life. With plants,
between their organs of generation. Hence, with animals, it will be the
interior structure which will determine the most important analogies:
with plants it will be the manner in which they fructify.[220]

"With animals we should look to nerves, organs of respiration, and those
of the circulation; with plants, to the embryo and its accessories, the
sexual organs of their flowers, &c.[221] To do this, will set us on to
the Natural Method, which is as it were a sketch traced by man of the
order taken by Nature in her productions.[222] Nevertheless the
divisions which we shall be obliged to establish, will still be
arbitrary and artificial, though presenting to our view sections
arranged in the order which Nature has pursued.[223]

"What, then," he asks,[224] "_is_ species--and can we show that species
has changed--however slowly?" He now covers some of the ground since
enlarged upon in Mr. Darwin's second chapter, in which the arbitrary
nature of the distinction between species and varieties is so well
exposed. "I shall show," says Lamarck (in substance, but I am compelled
to condense much), "that the habits by which we now recognize any
species, are due to the conditions of life [_circonstances_] under which
it has for a long time existed, and that these habits have had such an
influence upon the structure of each individual of the species, as to
have at length modified this structure, and adapted it to the habits
which have been contracted.[225]

"The individuals of any species," he continues, "certainly resemble
their parents; it is a universal law of nature that all offspring should
differ but little from its immediate progenitors, but this does not
justify the ordinary belief that species never vary. Indeed, naturalists
themselves are in continual difficulty as regards distinguishing species
from varieties; they do not recognize the fact that species are only
constant as long as the conditions in which they are placed are
constant. Individuals vary and form breeds which blend so insensibly
into the neighbouring species, that the distinctions made by naturalists
between species and varieties, are for the most part arbitrary, and the
confusion upon this head is becoming day by day more serious.[2