Especially
must this be so where the causation is complex. Hence we may infer that
the phenomena presented by the development of species, are not likely to
be truly conceived unless we keep in view the concrete agencies at work.
Let us look closely at the facts to be dealt with.

* * * * *

The growth of a thing is effected by the joint operation of certain
forces on certain materials; and when it dwindles, there is either a
lack of some materials, or the forces co-operate in a way different from
that which produces growth. If a structure has varied, the implication
is that the processes which built it up were made unlike the parallel
processes in other cases, by the greater or less amount of some one or
more of the matters or actions concerned. Where there is unusual
fertility, the play of vital activities is thereby shown to have
deviated from the ordinary play of vital activities; and conversely, if
there is infertility. If the germs, or ova, or seed, or offspring
partially developed, survive more or survive less, it is either because
their molar or molecular structures are unlike the average ones, or
because they are affected in unlike ways by surrounding agencies. When
life is prolonged, the fact implies that the combination of actions,
visible and invisible, constituting life, retains its equilibrium longer
than usual in presence of environing forces which tend to destroy its
equilibrium. That is to say, growth, variation, survival, death, if they
are to be reduced to the forms in which physical science can recognize
them, must be expressed as effects of agencies definitely
conceived--mechanical forces, light, heat, chemical affinity, &c.

This general conclusion brings with it the thought that the phrases
employed in discussing organic evolution, though convenient and indeed
needful, are liable to mislead us by veiling the actual agencies. That
which really goes on in every organism is the working together of
component parts in ways conducing to the continuance of their combined
actions, in presence of things and actions outside; some of which tend
to subserve, and others to destroy, the combination. The matters and
forces in these two groups, are the sole causes properly so called. The
words "natural selection," do not express a cause in the physical sense.
They express a mode of co-operation among causes--or rather, to speak
strictly, they express an effect of this mode of co-op