not thereby invalidated. Newton, Descartes, Fresnel, Carnot, Joule,
Mayer, Faraday, Helmholtz, Maxwell appear as one great succession of
unifiers. All have been engaged in the same work of consolidating
thought at the same time that they extended it. Their conceptions of
force, mass, matter, ether, atom, molecule have provisional validity as
the imagined objective substratum of our experience, and the fact that
we analyse these conceptions still further and sometimes discard them,
does not in any way invalidate the law or general form in which they
have enabled us to sum up our experience and predict the future.

But now we turn to the other side. In spite of the continued progress
noted on the mechanical side, it is true that the predominant scientific
interest changed in the nineteenth century from mechanics to biology,
from matter to life, from Newton to Darwin. Darwin was born in 1809, the
year in which Lamarck, who invented the term biology, published his
_Philosophie Zoologique_. The _Origin of Species_ appeared in 1858 after
the conservation of energy had been established, and the range and
influence of evolutionary biology have grown ever since.

Before anything can be said of the conclusions in this branch of science
one preliminary remark has to be made. From the philosophical point of
view the science of life includes all other, for man is a living animal,
and science is the work of his co-operating mind, one of the functions
of his living activity. What this involves on the philosophical side
does not concern us here, but it is necessary to indicate here the
nature of the contact between the two great divisions of science, the
mechanical and the biological, considered purely as sciences. For,
though we know that our consciousness as a function of life must in some
form come into the science of life, and is, in a sense, above it all, we
are yet able to draw conclusions, apparently of infinite scope, about
the behaviour of all living things around us and including ourselves,
just as we do about a stone or a star. And we are interested in this
chapter in seeing how this drawing of general conclusions keeps growing
with regard to the phenomena of life, just as it has grown with regard
to all other phenomena, and we have to consider what sort of difference
there is between the one class of generalizations and the other.

For those of us who are content to rest their conclusions on the
positively known, who, whi