ating.[66] Nevertheless in the present state of knowledge we
are still as a rule quite unable to connect cytological appearances
with any genetic consequences and save in one respect (obviously of
extreme importance--to be spoken of later) the two sets of phenomena
might, for all we can see, be entirely distinct.

I cannot avoid attaching importance to this want of connection between
the nuclear phenomena and the features of bodily organisation. All
attempts to investigate Heredity by cytological means lie under the
disadvantage that it is the nuclear changes which can alone be
effectively observed. Important as they must surely be, I have never
been persuaded that the rest of the cell counts for nothing. What we
know of the behaviour and variability of chromosomes seems in my
opinion quite incompatible with the belief that they alone govern
form, and are the sole agents responsible in heredity.[67]

If, then, progress was to be made in Genetics, work of a different
kind was required. To learn the laws of Heredity and Variation there
is no other way than that which Darwin himself followed, the direct
examination of the phenomena. A beginning could be made by collecting
fortuitous observations of this class, which have often thrown a
suggestive light, but such evidence can be at best but superficial and
some more penetrating instrument of research is required. This can
only be provided by actual experiments in breeding.

The truth of these general considerations was becoming gradually clear
to many of us when in 1900 Mendel's work was rediscovered.
Segregation, a phenomenon of the utmost novelty, was thus revealed.
From that moment not only in the problem of the origin of species, but
in all the great problems of biology a new era began. So unexpected
was the discovery that many naturalists were convinced it was untrue,
and at once proclaimed Mendel's conclusions as either altogether
mistaken, or if true, of very limited application. Many fantastic
notions about the workings of Heredity had been asserted as general
principles before: this was probably only another fancy of the same
class.

Nevertheless those who had a preliminary acquaintance with the facts
of Variation were not wholly unprepared for some such revelation. The
essential deduction from the discovery of segregation was that the
characters of living things are dependent on the presence of definite
elements or factors, which are treated as units in the pr