s, we can see that many of them
blend one into the other without presenting noteworthy differences at
any step."[228]

This has been well extended by Mr. Darwin in a passage which
begins:--"The affinities of all beings of the same class have sometimes
been represented by a great tree. I believe that this simile largely
speaks the truth."[229]

"What, then," continues Lamarck, "can be the cause of all this? Surely
the following: namely, that when individuals of any species change their
situation, climate, mode of existence, or habits [conditions of life],
their structure, form, organization, and in fact their whole being
becomes little by little modified, till in the course of time it
responds to the changes experienced by the creature."[230]

In his preface Lamarck had already declared that "the thread which gives
us a clue to the causes of the various phenomena of animal
organization, in the manifold diversity of its developments, is to be
found in the fact that Nature conserves in offspring all that their life
and environments has developed in parents." Heredity--"the hidden bond
of common descent"--tempered with the modifications induced by changed
habits--which changed habits are due to new conditions and
surroundings--this with Lamarck, as with Buffon and Dr. Darwin, is the
explanation of the diversity of forms which we observe in nature. He now
goes on to support this--briefly, in accordance with his design--but
with sufficient detail to prevent all possibility of mistake about his
meaning.

"In the same climate differences in situation, and a greater or less
degree of exposure, affect simply, in the first instance, the
individuals exposed to them; but in the course of time, these repeated
differences of surroundings in individuals which reproduce themselves
continually under similar circumstances, induce differences which become
part of their very nature; so that after many successive generations,
these individuals, which were originally, we will say, of any given
species, become transformed into a different one."[231]

"Let us suppose that a grass growing in a low-lying meadow gets carried
by some accident to the brow of a neighbouring hill, where the soil is
still damp enough for the plant to be able to exist. Let it live here
for many generations, till it has become thoroughly accustomed to its
position, and let it then gradually find its way to the dry and almost
arid soil of a mountain side; if the pla