not suffice for the examination of one-fiftieth part of all these."
That is true, but then comes the great convenience of the way things
are arranged; which is, that although there are these immense numbers
of different kinds of living things in existence, yet they are built
up, after all, upon marvellously few plans.

There are certainly more than 100,000 species of insects, and yet
anybody who knows one insect--if a properly chosen one--will be able
to have a very fair conception of the structure of the whole. I do not
mean to say he will know that structure thoroughly, or as well as it is
desirable he should know it; but he will have enough real knowledge to
enable him to understand what he reads, to have genuine images in his
mind of those structures which become so variously modified in all the
forms of insects he has not seen. In fact, there are such things as
types of form among animals and vegetables, and for the purpose of
getting a definite knowledge of what constitutes the leading
modifications of animal and plant life, it is not needful to examine
more than a comparatively small number of animals and plants.

Let me tell you what we do in the biological laboratory which is lodged
in a building adjacent to this. There I lecture to a class of students
daily for about four-and-a-half months, and my class have, of course,
their text-books; but the essential part of the whole teaching, and
that which I regard as really the most important part of it, is a
laboratory for practical work, which is simply a room with all the
appliances needed for ordinary dissection. We have tables properly
arranged in regard to light, microscopes, and dissecting instruments,
and we work through the structure of a certain number of animals and
plants. As, for example, among the plants, we take a yeast plant, a
_Protococcus_, a common mould, a _Chara_, a fern, and some