Two objections to the employment of these analogies will present
themselves at once. The definition may be all very well as far as the
machines are concerned, but, it may be asked, should a living thing like a
horse or a dog be compared with the steamship or the locomotive? Can we
look upon the living thing as a mechanism in the proper sense of the word?
A second objection will be that human invention and ingenuity have
controlled the evolution of the steamship and engine by the perfection of
newer and more efficient parts. It is certainly true that organic
evolution cannot be controlled in the same way by men, and that science
has not yet found out what all the factors are. And yet we are going to
learn in a later discussion that nature's method of transforming organisms
in the course of evolution is strikingly similar to the human process of
trial and error which has brought the diverse modern mechanisms to their
present conditions of efficiency. This matter, however, must remain for
the time just as it stands. The first objection, namely, that an organism
ought not to be viewed as a machine, is one that we must meet immediately,
because it is necessary at the very outset to gain a clear idea of the
essentially mechanical nature of living things and of their relations to
the conditions under which they live. It is only when we have such a clear
understanding that we can profitably pursue the further inquiries into the
evidence of evolution. Our first real task, therefore, is an inquiry into
certain fundamental questions about life and living things, upon which we
shall build as we proceed.

* * * * *

All living things possess three general properties which seem to be
unique; these are a peculiar chemical constitution, the power of repairing
themselves as their tissues wear out, and the ability to grow and