n the doctrine of natural
selection. Huxley (1863) followed with an application of the law to man,
and in 1866 I gave a comprehensive sketch of its application throughout
the whole animal world. In 1874 I published the first edition of the
present work.

The doctrine of evolution is now a vital part of biology, and we might
accept the evolution of man as a special deduction from the general law.
Three great groups of evidence impose that law on us. The first group
consists of the facts of palaeontology, or the fossil record of past
animal life. Imperfect as the record is, it shows us a broad divergence
of successively changing types from a simple common root, and in some
cases exhibits the complete transition from one type to another. The
next document is the evidence of comparative anatomy. This science
groups the forms of living animals in such a way that we seem to have
the same gradual divergence of types from simple common ancestors. In
particular, it discovers certain rudimentary organs in the higher
animals, which can only be understood as the shrunken relics of organs
that were once useful to a remote ancestor. Thus, man has still the
rudiment of the third eyelid of his shark-ancestor. The third document
is the evidence of embryology, which shows us the higher organism
substantially reproducing, in its embryonic development, the long
series of ancestral forms.

_II.--Man's Embryonic Development_

The first stage in the development of any animal is the tiny speck of
plasm, hardly visible to the naked eye, which we call the ovum, or
egg-cell. It is a single cell, recalling the earliest single-celled
ancestor of all animals. In its immature form it is not unlike certain
microscopic animalcules known as _amoeboe_. In its mature form it is
about 1/125th of an inch in diameter.

When the male germ has blended with the female in the ovum, the new cell
slowly divides into two, with a very complicated division of the
material composing its nucleus. The two cells divide into four, the four
into eight, and so on until we have a round cluster of cells, something
like a blackberry in shape.

This _morula_, as I have called it, reproduces the next stage in the
development of life. As all animals pass through it, our biogenetic law
forces us to see in it an ancestral stage; and in point of fact we have
animals of this type living in Nature to-day. The round cluster becomes
filled with fluid, and we have a hollow sphere