rain and spinal cord, and they find
that the nomenclature which fits the one answers for the other. They
carry their microscopic inquiries in the case of the dog as far as they
can, and they find that his body is resolvable into the same elements as
those of the man. Moreover, they trace back the dog's and the man's
development, and they find that, at a certain stage of their existence,
the two creatures are not distinguishable the one from the other; they
find that the dog and his kind have a certain distribution over the
surface of the world, comparable in its way to the distribution of the
human species. What is true of the dog they tell us is true of all the
higher animals; and they assert that they can lay down a common plan for
the whole of these creatures, and regard the man and the dog, the horse
and the ox as minor modifications of one great fundamental unity.
Moreover, the investigations of the last three-quarters of a century
have proved, they tell us, that similar inquiries, carried out through
all the different kinds of animals which are met with in nature, will
lead us, not in one straight series, but by many roads, step by step,
gradation by gradation, from man, at the summit, to specks of animated
jelly at the bottom of the series. So that the idea of Leibnitz, and of
Bonnet, that animals form a great scale of being, in which there are a
series of gradations from the most complicated form to the lowest and
simplest; that idea, though not exactly in the form in which it was
propounded by those philosophers, turns out to be substantially correct.
More than this, when biologists pursue their investigations into the
vegetable world, they find that they can, in the same way, follow out
the structure of the plant, from the most gigantic and complicated trees
down through a similar series of gradations, until they arrive at specks
of animated jelly, which they are puzzled to distinguish from those
specks which they reached by the animal road.

Thus, biologists have arrived at the conclusion that a fundamental
uniformity of structure pervades the animal and vegetable worlds, and
that plants and animals differ from one another simply as diverse
modifications of the same great general plan.

Again, they tell us the same story in regard to the study of function.
They admit the large and important interval which, at the present time,
separates the manifestations of the mental faculties observable in the
higher form