attack the Species-problem with any hope of success there are vast
arrears to be made up. He would be a bold man who would now assert that
there was no sense in which the term Species might not have a strict and
concrete meaning in contradistinction to the term Variety. We have been
taught to regard the difference between species and variety as one of
degree. I think it unlikely that this conclusion will bear the test of
further research. To Darwin the question, What is a variation? presented
no difficulties. Any difference between parent and offspring was a
variation. Now we have to be more precise. First we must, as de Vries
has shown, distinguish real, genetic, variation from FLUCTUATIONAL
variations, due to environmental and other accidents, which cannot
be transmitted. Having excluded these sources of error the variations
observed must be expressed in terms of the factors to which they are due
before their significance can be understood. For example, numbers of the
variations seen under domestication, and not a few witnessed in nature,
are simply the consequence of some ingredient being in an unknown way
omitted from the composition of the varying individual. The variation
may on the contrary be due to the addition of some new element, but to
prove that it is so is by no means an easy matter. Casual observation
is useless, for though these latter variations will always be dominants,
yet many dominant characteristics may arise from another cause, namely
the meeting of complementary factors, and special study of each case
in two generations at least is needed before these two phenomena can be
distinguished.

When such considerations are fully appreciated it will be realised that
medleys of most dissimilar occurrences are all confused together under
the term Variation. One of the first objects of genetic analysis is to
disentangle this mass of confusion.

To those who have made no study of heredity it sometimes appears that
the question of the effect of conditions in causing variation is one
which we should immediately investigate, but a little thought will
show that before any critical inquiry into such possibilities can be
attempted, a knowledge of the working of heredity under conditions as
far as possible uniform must be obtained. At the time when Darwin was
writing, if a plant brought into cultivation gave off an albino variety,
such an event was without hesitation ascribed to the change of life. Now
we see that