himself--a promise which perhaps is still before us, but the
fulfillment of which is yet far off. As time has gone on, the pursuit of
natural knowledge has seemed to lead man away from himself into the
furthermost parts of the universe, and into secret workings of Nature in
which he appears to be of little or no account; and his knowledge of the
nature of living things, and so of his own nature, has advanced slowly,
waiting till the progress of other branches of natural knowledge can bring
it aid. Yet in the past hundred years the biologic sciences, as we now
call them, have marched rapidly onward.

We may look upon a living body as a machine doing work in accordance with
certain laws, and may seek to trace out the working of the inner wheels:
how these raise up the lifeless dust into living matter, and let the
living matter fall away again into dust, giving out movement and heat. Or
we may look upon the individual life as a link in a long chain, joining
something which went before to something about to come, a chain whose
beginning lies hid in the farthest past, and may seek to know the ties
which bind one life to another. As we call up to view the long series of
living forms, living now or flitting like shadows on the screen of the
past, we may strive to lay hold of the influences which fashion the
garment of life. Whether the problems of life are looked upon from the one
point of view or the other, we to-day, not biologists only, but all of us,
have gained a knowledge hidden even from the philosophers a hundred years
ago.

Of the problems presented by the living body viewed as a machine, some may
be spoken of as mechanical, others as physical, and yet others as
chemical, while some are, apparently at least, none of these. In the
seventeenth century William Harvey, laying hold of the central mechanism
of the blood stream, opened up a path of inquiry which his own age and the
century which followed trod with marked success. The knowledge of the
mechanism of the animal and of the plant advanced apace, but the physical
and chemical problems had yet to wait. The eighteenth century, it is true,
had its physics and its chemistry; but, in relation at least to the
problems of the living being, a chemistry which knew not oxygen and a
physics which knew not the electricity of chemical action were of little
avail. The philosopher of 1799, when he discussed the functions of the
animal or of the plant involving chemical changes, w