12 DOCTRINE OF EVOLUTION 



The several kinds are no more interchangeable than are 

 the different forms of locomotives that we have men- 

 tioned. 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 modifica- 

 tion. 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 modifica- 

 tions 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 hi 

 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 per- 

 fection 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 trans- 

 forming organisms in the course of evolution is strikingly 



