Finally, we must, in my opinion, count among the chief inductive bases
of the theory of evolution the foetal development of the individual
organism, the whole science of embryology or ontogeny. But as the
later chapters will deal with this in detail, I need say nothing
further here. I shall endeavour in the following pages to show, step
by step, how the whole of the embryonic phenomena form a massive chain
of proof for the theory of evolution; for they can be explained in no
other way. In thus appealing to the close causal connection between
ontogenesis and phylogenesis, and taking our stand throughout on the
biogenetic law, we shall be able to prove, stage by stage, from the
facts of embryology, the evolution of man from the lower animals.

The general adoption of the theory of evolution has definitely closed
the controversy as to the nature or definition of the species. The
word has no ABSOLUTE meaning whatever, but is only a group-name, or
category of classification, with a purely relative value. In 1857, it
is true, a famous and gifted, but inaccurate and dogmatic, scientist,
Louis Agassiz, attempted to give an absolute value to these
"categories of classification." He did this in his Essay on
Classification, in which he turns upside down the phenomena of organic
nature, and, instead of tracing them to their natural causes, examines
them through a theological prism. The true species (bona species) was,
he said, an "incarnate idea of the Creator." Unfortunately, this
pretty phrase has no more scientific value than all the other attempts
to save the absolute or intrinsic value of the species.

The dogma of the fixity and creation of species lost its last great
champion when Agassiz died in 1873. The opposite theory, that all the
different species descend from common stem-forms, encounters no
serious difficulty to-day. All the endless research into the nature of