are congenital and thus born with the individual and transmitted by inheritance.  Clearly enough those animals that have a variation which makes them a little better adapted for the struggle will be the ones to live and hence to produce offspring, while those without such advantage will be the ones to die.  We may suppose, for example, that some of the individuals had longer necks than the average.  In time of scarcity of food these individuals would be able to get food that the short-necked individuals could not reach.  Hence in times of famine the long-necked individuals would be the ones to survive.  Now if this peculiarity were a congenital variation it would be already represented in the germ plasm, and consequently it would be inherited by the next generation.  The short-necked individuals being largely destroyed in this struggle for food, it would follow that the next generation would be a little better off than the last, since all would inherit this tendency toward a long neck.  A few generations would then see the disappearance of all individuals which did not show either this or some other corresponding advantage, and in this way the lengthened neck would be added permanently as a part of the machine.  When this time came this peculiarity would no longer give its possessors any advantage over its rivals, since all would possess it.  Now, therefore, some new variation would in the same way determine which animals should live and which should die in the struggle, and in time a new modification would be added to the machine.  And thus this process continues, one variation after another being added, until the machine is slowly built into a more and more complicated structure, always active but with a constantly increasing efficiency.  The construction is a natural one.  A mixing of germ plasm in sexual reproduction or some other agencies produce congenital variations; natural selection acting upon the numerous progeny selects the best of the new variations, and heredity preserves and hands them down to posterity.

All students of whatever school recognize the force of this principle and look upon natural selection as an efficient agency in machine building.  It is probably the most fundamental of the external laws that have guided the process.  There are, however, certain other laws which have played a more or less subordinate part.  The chief of these are the influence of migration and isolation, and the direct influence of the environment.  Each of these laws has its own school of advocates, and each has been given by its advocates the chief role in the process of machine building.