192 THE STORY OF LIFE'S MECHANISM. 



Here, then, we have before us the factors 

 which have been concerned in the building of 

 the living machine under nature's hands. Repro- 

 duction keeps in existence a constantly active, 

 unstable, readily modified organism as a basis 

 upon which to build. Variation offers constantly 

 new modifications of the type, while heredity 

 insures that the modifications produced in the 

 machine by the influences which give rise to the 

 variations shall be permanently fixed. 



METHOD OF MACHINE BUILDING. 



Natural Selection. The method by which these 

 factors have worked together to build up the 

 living machines is easily understood in its general 

 aspects, although there are many details as yet 

 unsolved. The general facts connected with the 

 evolution of animals are matters of common 

 knowledge. We need do no more than outline 

 the subject, since it is well understood by all. 

 The basis of the method is natural selection, 

 which acts in this machine building something 

 as follows : 



The law of reproduction, as we have seen, pro- 

 duces new individuals with extraordinary rapidity, 

 and as a result more individuals are born than 

 can possibly find sustenance in the world. Hence 

 only a few of the offspring of any animal or plant 

 can live long enough to produce offspring in turn. 

 The many must die that the few may live ; and 

 there is, therefore, a constant struggle among the 

 individuals that are born for food or for room in 

 the world. In this struggle for existence of course 



