are introduced into the pages of
this work. So long as these generalizations, regarded as first
attempts to deduce "laws" in the form of "generalized statement of
facts based observation," are properly introduced into an elementary
text-book, intended for the isolated worker cut off from the lecture
room, their intercalation is both healthy and desirable.

Mr. Wells has kept these precepts constantly in mind in the
preparation of his work, and in the formulation of his plans for its
future extension, thereby enhancing the value of the book itself, and
at the same time, discouraging the system of pure cram, which is
alien to the discipline of biological science.

G. B. Howes
Royal College of Science,
South Kensington;
November 30, 1892.

-Preface_

No method of studying-- more especially when the objects of study
are tangible things-- can rival that prosecuted under the direction and
in the constant presence of a teacher who has also a living and vivid
knowledge of the matter which he handles with the student. In the
ideal world there is a plentiful supply of such teachers, and easy
access to their teaching, but in this real world only a favoured few
enjoy these advantages. Through causes that cannot be discussed
here, a vast number of solitary workers are scattered through the
country, to whom sustained help in this form is impossible, or
possible only in days stolen from a needed vacation; and to such
students especially does this book appeal, as well as to those more
fortunate learners who are within reach of orderly instruction, but
anxious to save their teachers' patience and their own time by some
preliminary work.

One of the most manifest disadvantages of book-work, under the
conditions of the solitary worker, is the rigidity of its expressions; if
the exact meaning is doubtful, he can not ask a question. This has
been kept in view throughout; the writer has, above all, sought to be
explicit-- has, saving over-sights, used no uncommon or technical
term without a definition or a clear indication of its meaning.

In this study of Biology, the perception and memory of form is a very
important factor indeed. Every student should draw sketches of his
dissections, and accustom himself to copying book diagrams, in
order to train his eye to perception of details he might otherwise
disregard. The drawing required is within the reach of all; but for
those who are very inexperienced, tracing figures is a usef