is not the obvious resemblance of the embryos of different animals,
and their general similarity to our extinct ancestors in this or that
organ, on which we must rest our case. A careful study must be made of
the various stages through which all embryos pass, and an effort made
to prove their real identity and therefore genealogical relation.

This is a task of great subtlety and delicacy. Many scientists have
worked at it together with Professor Haeckel--I need only name our own
Professor Balfour and Professor Ray Lankester--and the scheme is
fairly complete. But the general reader must not expect that even so
clear a writer as Haeckel can describe these intricate processes
without demanding his very careful attention. Most of the chapters in
the present volume (and the second volume will be less difficult) are
easily intelligible to all; but there are points at which the line of
argument is necessarily subtle and complex. In the hope that most
readers will be induced to master even these more difficult chapters,
I will give an outline of the characteristic argument of the work.
Haeckel's distinctive services in regard to man's evolution have been:

1. The construction of a complete ancestral tree, though, of course,
some of the stages in it are purely conjectural, and not final.

2. The tracing of the remarkable reproduction of ancestral forms in
the embryonic development of the individual. Naturally, he has not
worked alone in either department.

The second volume of this work will embody the first of these two
achievements; the present one is mainly concerned with the latter. It
will be useful for the reader to have a synopsis of the argument and
an explanation of some of the chief terms invented or employed by the