t 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.

==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 wo