h it, one brings
in something entirely different from the direct action of light. One
implicitly attributes to organized matter a certain capacity _sui
generis_, the mysterious power of building up very complicated machines
to utilize the simple excitation that it undergoes.

But this is just what is claimed to be unnecessary. Physics and
chemistry are said to give us the key to everything. Eimer's great work
is instructive in this respect. It is well known what persevering effort
this biologist has devoted to demonstrating that transformation is
brought about by the influence of the external on the internal,
continuously exerted in the same direction, and not, as Darwin held, by
accidental variations. His theory rests on observations of the highest
interest, of which the starting-point was the study of the course
followed by the color variation of the skin in certain lizards. Before
this, the already old experiments of Dorfmeister had shown that the same
chrysalis, according as it was submitted to cold or heat, gave rise to
very different butterflies, which had long been regarded as independent
species, _Vanessa levana_ and _Vanessa prorsa_: an intermediate
temperature produces an intermediate form. We might class with these
facts the important transformations observed in a little crustacean,
_Artemia salina_, when the salt of the water it lives in is increased or
diminished.[32] In these various experiments the external agent seems to
act as a cause of transformation. But what does the word "cause" mean
here? Without undertaking an exhaustive analysis of the idea of
causality, we will merely remark that three very different meanings of
this term are commonly confused. A cause may act by _impelling_,
_releasing_, or _unwinding_. The billiard-ball, that strikes another,
determines its movement by _impelling_. The spark that explodes the
powder acts by _releasing_. The gradual relaxing of the spring, that
makes the phonograph turn, _unwinds_ the melody inscribed on the
cylinder: if the melody which is played be the effect, and the relaxing
of the spring the cause, we must say that the cause acts by _unwinding_.
What distinguishes these three cases from each other is the greater or
less solidarity between the cause and the effect. In the first, the
quantity and quality of the effect vary with the quantity and quality of
the cause. In the second, neither quality nor quantity of the effect
varies with quality and quantity o