l construction and development,
and of other classes of zooelogical facts, and then he must turn his
attention from the dead object of laboratory analysis to the workings of
organic machines. The way an organism lives its life in dynamic relations
to the varied conditions of existence, as well as the mutual physiological
relations of the manifold parts of a single organism, reveal certain
definite natural forces at work. Therefore his next task is to compare the
results accomplished by these factors in the brief time they may be seen
in operation with the products of the whole process of organic evolution,
to learn, like the geologist in his sphere, that the present-day natural
forces are able to do what reason says they have done in the past.

When the subject of inquiry was the reality of evolution, it was perhaps
surprising to find that even the most familiar animals like cats and frogs
provided adequate data for science to use in formulating its principles.
So it is with the matter of method; it is unnecessary to go beyond the
observations of a day or a week of human life to find forces at work, as
real and vital as animal existence and organic life themselves. This is
true, because evolution is true, and because the lives of all creatures
follow one consistent law. Our task is therefore much more simple than
most people suppose it to be; let us look about us and classify what we
may observe, increasing our knowledge from the wide array of equally
natural facts supplied by the biologist.

The analogies of the steamship and the locomotive proved useful at many
times during the discussion of the fact of evolution, and even in the
present connection they will still be of service. The evolution of these
dead machines has been brought about by man, who, as an element of their
environment, has been their creator as well as the director of their
historical transformations. The result of their changes has been greater
efficiency and better adjustment or adaptation to certain requirements
fixed by man himself. The whole process of improvement has been one, in
brief, of trial and error; new inventions have often been worthless, and
they have been relegated to the scrap-heap, while the better part has been
finally incorporated in the type machine. In brief, then, the important
elements in the evolution of these examples have been three; first,
_adaptation_, second, the _origination of new parts_, and third, the
_retention of