The several kinds are no more interchangeable than are the different forms of locomotives that we have mentioned.  The flat-bottom boat of the Mississippi would not venture to cross the Atlantic Ocean in winter, nor would the “Lusitania” attempt to plow a way up the shallow mud-banked Mississippi.  These products of mechanical development are not efficient unless they run under the circumstances which have controlled their construction, unless they are fitted or adapted to the conditions under which they must operate.

Evolution, then, means descent with adaptive modification.  We must examine the various kinds of living creatures everywhere to see if they, like the machines, exhibit in their make-up similar elements which indicate their common ancestry in an earlier age, and if we can interpret their differences as the results of modifications which fit them to occupy different place in nature.

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.

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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 multiply.  The third property is so familiar that we fail to see how sharply it distinguishes the creatures of the organic world.  To realize this we have only to imagine how strange it would seem if locomotives