under certain conditions certain kinds of acquired characters may be inherited, although this is still disputed by others.  Into this discussion we cannot enter here.  The reason for referring to it at all is, however, evident.  We are searching for nature’s method of building machines.  It is perfectly clear that variations among animals and plants are the foundations of the successive steps in advance made in this machine building, but of course only such variations as can be transmitted to posterity can serve any purpose in this development.  If therefore it should prove that acquired characters can not be inherited, then we should no longer be able to look upon the direct influence of the surroundings as a factor in the machine building.  We should then have nothing left except the congenital variations produced by sexual union, or the direct variation of the germ plasm as a factor for advance.  If, however, it shall prove that acquired characters may even occasionally be inherited, then the direct effect of the environment upon the individual will serve as a decided assistance in our problem.

Here, then, we have before us the factors which have been concerned in the building of the living machine under nature’s hands.  Reproduction 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, produces 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 the weakest will go to the wall, while those that are best adapted for their place in life will be the ones to get food, live, and reproduce their kind.  This is at all events true among the lower animals, although with mankind the law hardly applies.  Now, among the individuals that are born there will be no two exactly alike, since variations are universal, many of which