beings, by endeavouring to find physical causes competent to effect that
change of one species into another, which De Maillet had only supposed to
occur. And Lamarck conceived that he had found in Nature such causes, amply
sufficient for the purpose in view. It is a physiological fact, he says,
that organs are increased in size by action, atrophied by inaction; it is
another physiological fact that modifications produced are transmissible to
offspring. Change the actions of an animal, therefore, and you will change
its structure, by increasing the development of the parts newly brought
into use and by the diminution of those less used; but by altering the
circumstances which surround it you will alter its actions, and hence, in
the long run, change of circumstance must produce change of organisation.
All the species of animals, therefore, are, in Lamarck's view, the result
of the indirect action of changes of circumstance, upon those primitive
germs which he considered to have originally arisen, by spontaneous
generation, within the waters of the globe. It is curious, however, that
Lamarck should insist so strongly [Footnote: See _Phil. Zoologique_,
vol. i. p. 222. et seq.] as he has done, that circumstances never in any
degree directly modify the form or the organisation of animals, but only
operate by changing their wants and consequently their actions; for he
thereby brings upon himself the obvious question, How, then, do plants,
which cannot be said to have wants or actions, become modified? To this he
replies, that they are modified by the changes in their nutritive
processes, which are effected by changing circumstances; and it does not
seem to have occurred to him that such changes might be as well supposed to
take place among animals.

When we have said that Lamarck felt that mere speculation was not the way
to arrive at the origin of species, but that it was necessary, in order to
the establishment of any sound theory on the subject, to discover by
observation or otherwise, some _vera causa_, competent to give rise to