mixture of mechanistic "explanations" and
vitalistic hypothesis!

In his third law, that use and disuse are powerful to modify organs,
Lamarck is upon more solid ground, and can point to many instances of
the visible effect of these factors of change. It is of course rather
closely bound up with his second law and may even be regarded as an
extension of it.

The law has reference to one of the most powerful means employed by
Nature to diversify species, a means which comes into play whenever the
environment changes. The cause of the great diversity shown by animal
species is indeed ultimately to be sought in the environment. As the
imperfect and earliest forms developed they spread over the earth and
invaded the utmost corners of it:--"One can imagine what an enormous
variety of habitats, stations, climates, available foods, environing
media, etc., animals and plants have had to endure, as the existing
species were forced to change their place of abode. And although these
changes have taken place with extreme slowness ... their reality,
necessitated by various causes, has none the less induced the species
affected by them slowly to change their manner of life and their
habitual actions. Through the effects of the second and third of the
laws cited above, these induced activity-changes must have brought into
being new organs, and must have been able to develop them further if
more frequent use was made of them; they must in the same way have been
capable of bringing about the degeneration and finally the complete
disappearance of existing organs which had become useless" (p. 161).

On the other hand, if the environment does not change, species remain
constant.

It is to be noted that change in environment is rather the occasion than
the cause of modification; the environment induces the organism to
change its habitual way of life; it sets up new needs, to satisfy which
the organism must modify its structure. It is the organism that takes
the active part in all this, the action of the environment is indirect.

Of Lamarck's fourth law, which asserts the transmission of acquired
characters, little need here be said in the way of exposition. Upon the
truth of it depends of course Lamarck's whole theory. He himself never
dreamed that anyone would ever dispute it.

Lamarck sums up as follows:--"By the four laws which I have just
enunciated all the facts of organisation seem to me to be easily
explained; the progression