==Migration and Isolation.==—­The production of the various types of machines has been undoubtedly facilitated by the migrations of animals and the isolation of different groups of descendants from each other by various natural barriers.  The variations which occur in organisms are so great that they would sometimes run into abnormal structures were it not for the fact that sexual reproduction constantly tends to reduce them.  In an open country where animals and plants interbreed freely, it will commonly happen that individuals with certain peculiarities will mate with others without such peculiarities, and the offspring will therefore inherit the peculiarity not in increased degree but in decreased degree.  This constant interbreeding of individuals will tend to prevent the formation of many modifications in the machine which become started by variations.  Now plainly if some such individuals, with a peculiar variation, should migrate into a new territory or become isolated from their relatives which do not have similar variations, these individuals will be obliged to breed with each other.  The result will be that the next generation, arising thus from two parents each of which shows the same variation, will show it also in equal or increased degree.  Migrations and isolations will thus tend to fix in the machine variations which sexual union or other influences inaugurate.  Now in the history of the earth’s surface there have been many changes which tend to bring about such migration and isolations, and this factor has doubtless played a more or less important part in the building of the machines.  How great a part we cannot say, nor is it necessary for our purpose to decide; for in all these cases the machine building has only been the result of the hereditary transmission of congenital variation under certain peculiar conditions.  The fundamental process is the same as already considered, only the details of its working being in question.

==Direct Influence of the Environment.==—­Under this head we have a subject of great importance.  It is an undoubted fact that the environment has a very decided effect upon the machine.  These direct effects of the environment are very positive and in great variety.  The tropical sun darkens the human skin; cold climate stunts the growth of plants; lack of food dwarfs all animals and plants, and hundreds of other similar examples could be selected.  Another class of similar influences are those produced by use and disuse.  Beyond question the use of an organ tends to increase its size, and disuse to decrease it.  Combats of animals with each other tend to increase their strength, flight from enemies their running powers, etc.