on the heritability of ‘acquired characters’ was one of the most important towards the end of last century, and although the majority of biologists now follow Weismann 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 radio-activity.

An important consequence of the advance in our knowledge of heredity associated with the work of Mendel and his successors is a tendency to doubt whether natural selection is of such fundamental importance in shaping the course of evolution as was supposed in the years of the first enthusiasm which followed the publication of the Origin.