as it appears to us, one which tends to produce a regular
increase in complexity, and one which disturbs and diversifies this
regular advance.

The first cause Lamarck calls the vital power (_pouvoir de la vie_); the
other may be called the influence of circumstance (_Anim. s. Vert._, p.
134). To the latter cause are due the lacunae, the blind alleys, and the
complications which the otherwise simple scale of perfection shows.

To explain both these aspects of evolution Lamarck propounded in his
volume of 1816 four laws, which read as follows:--

"_First Law_.--Life, by its own forces, tends continually to increase
the volume of every body possessing it, and to extend the dimensions of
its parts, up to a limit which it brings about itself.

"_Second Law_.--The production of a new organ in an animal body results
from the arisal and continuance of a new need, and from the new movement
which this need brings into being and sustains.

"_Third Law_.--The degree of development of organs and their force of
action are always proportionate to the use made of these organs.

"_Fourth Law_.--All that has been acquired, imprinted or changed in the
organisation of the individual during the course of its life is
preserved by generation and transmitted to the new individuals that
descend from the individual so modified" (pp. 151-2).

It is mainly but not entirely by reason of the first of these laws that
organisation tends to progress, and mainly by reason of the second and
third that difference of environment brings about diversity of
organisation. In virtue of the fourth law the acquirements of the
individual become the property of the race.

Lamarck's exposition of his first law, that life tends by its own powers
to enlarge and extend its bodily instrument, is vague and difficult to
understand. He has already explained some pages back how the first
organisms arose by spontaneous generation in the form of minute
gelatinous utricles (_cf._ Oken). He conceives that it is in the
movements of the fluids proper to the organism that the power resides to
enlarge and extend the body. Nutrition alone is not sufficient to bring
about extension; a special force is required, acting from within
outwards (p. 153). In the most primitive organisms the movements of the
vital fluids are weak and slow, but in the course of evolution they
gradually accelerate, and, becoming more rapid, trace out canals in the
delicate tissue which contains them,