the light loads of suburban traffic, nor do we expect to see a little
switch engine attempt to draw "the Twentieth Century Limited" to Chicago.
In the evolution, then, of modern locomotives, differences have come
about, even though the common ancestor is one single type; and these
differences have an adaptive value to certain specific conditions. A
second illustration will be useful. Fulton's steamboat of just a century
ago was in a certain true sense the ancestor of the "Lusitania," with its
deep keel and screw propellers, of the side-wheel steamship for river and
harbor traffic like the "Priscilla," of the stern-wheel flat-bottom boats
of the Mississippi, and of the battleship, and the tug boat. As in the
first instance, we know that each modern type has developed through the
accumulation of changes, which changes are likewise adjustments to
different conditions. The diversity of modern types of steamships may be
attributed therefore to adaptation.

The several kinds are no more interchangeable than are the different forms
of locomotives that we have mentioned. The flat-bottom boat of the
Mississippi would not venture to cross the Atlantic Ocean in winter, nor
would the "Lusitania" attempt to plow a way up the shallow mud-banked
Mississippi. These products of mechanical development are not efficient
unless they run under the circumstances which have controlled their
construction, unless they are fitted or adapted to the conditions under
which they must operate.

Evolution, then, means _descent with adaptive modification_. We must
examine the various kinds of living creatures everywhere to see if they,
like the machines, exhibit in their make-up similar elements which
indicate their common ancestry in an earlier age, and if we can interpret
their differences as the results of modifications which fit them to occupy
different place in nature.