mann in so far as they deny that
'acquired' characters are transmissible, the question is not yet
completely settled; all that can be said is that, in spite of many
attempts to prove the contrary, there is no satisfactory evidence of the
transmission to offspring of effects impressed on the body of the
parent, unless the germ-cells themselves have been affected by the same
cause--as for example in some cases of long-continued poisoning by
alcohol or similar drugs.

While the problem of the transmission of acquired characters, and of the
cause of variation and its relation to evolution, was occupying much of
the attention of biologists, the whole problem entered upon a new phase
in the year 1900 with the re-discovery of Mendel's work on heredity.
Mendel worked with plants, and published his results in 1865, but at
that time the biological world was too much occupied with the fierce
controversy which raged over _The Origin of Species_ to take much notice
of a paper the bearing of which upon it was not appreciated. Mendel's
discovery never came to the notice of Darwin, was buried in an obscure
periodical, and remained unknown until many years after the death of its
author. In 1900 it was unearthed, and, largely owing to the work of
Bateson, it rapidly became known as one of the most important
contributions to Biology made during the period under review.

This is not the place to describe in detail the nature of Mendel's
theory. Its essence is, firstly, that the various characteristics of an
organism are in general inherited quite independently of one another;
and, secondly, that the germ-cells of a hybrid are pure in respect of
any one character, that is to say, that any one germ-cell can only
transmit any unit character as it was received from one parent or the
other, and not a combination of the two. This leads to a conception of
the organism as something like a mosaic, in which each piece of the
pattern is transmitted in inheritance independently of the rest, and in
which any piece cannot be modified by association with a different but
corresponding piece derived from another ancestor. It is impossible to
say as yet whether this conception at all completely represents the
nature of the living organism, but it is one which is exercising
considerable influence in biological thought, and if established it will
mark a revolution in Biology hardly inferior to that brought about in
Physics and Chemistry by the discovery of r