The greatest contribution which modern naturalists have made to
knowledge concerns the origin of organic species. The students of a
century ago believed that all these different kinds had been suddenly
created either through natural law or by the immediate will of God. We
now know that from the beginning of organic life in the remote past to
the present day one kind of animal or plant has been in a natural and
essentially gradual way converted into the species which was to be its
successor, so that all the vast and complicated assemblage of kinds
which now exists has been derived by a process of change from the
forms which in earlier ages dwelt upon this planet. The exact manner
in which these alterations were produced is not yet determined, but in
large part it has evidently been brought about by the method indicated
by Mr. Darwin, through the survival of the fittest individuals in the
struggle for existence.

Until men came to have a clear conception as to the spherical form of
the earth, it was impossible for them to begin any intelligent
inquiries concerning its structure or history. The Greeks knew the
earth to be a sphere, but this knowledge was lost among the early
Christian people, and it was not until about four hundred years ago
that men again came to see that they dwelt upon a globe. On the basis
of this understanding the science of geology, which had in a way been
founded by the Greeks, was revived. As this science depends upon the
knowledge which we have gained of astronomy, physics, chemistry, and
biology, all of which branches of learning have to be used in
explaining the history of the earth, the advance which has been made
has been relatively slow. Geology as a whole is the least perfectly
organized of all the divisions of learning. A special difficulty
peculiar to this science has also served to hinder its development.
All the other branches of learning deal mainly, if not altogether,