onder at it is because, like other wonderful yet everyday
facts, such as the growth of a great tree from a tiny seed, it _is_ so
everyday that we have ceased to wonder at it. It is there: we know that.
But have we any kind of idea how it comes about? The duck does not, as a
matter of common experience, come out of a hen's egg. Why does it come
out of a duck's egg? Why doesn't it come out, if only rarely, from a
hen's egg? In other words, do we know what it is that explains
inheritance or how it is that there is such a thing as inheritance?
Well, candour obliges me to say that we do not. In spite of all the work
which has been expended upon this question we are totally ignorant of
the mechanism of heredity. Nevertheless it will be instructive to glance
at the theories which have been put forward to explain this matter.

All living things spring from a small germ, and in the vast majority of
cases this germ is the product in part of the male and in part of the
female parent. It is therefore natural that we should in the first place
turn our attention to this germ and ask ourselves whether there is
anything in its construction which will give us the key of the mystery.
There is not, at least there is nothing definite as shown by our most
powerful microscopes. To be sure there is a remarkable substance, called
chromatin because of its capacity for taking up certain dyes, which
evidently plays some profoundly important part in the processes of
development. We may suspect that this is the thing which carries the
physical characteristics from one generation to another, but we cannot
prove it; and though some authorities think that it is, others deny it.
Even if it be, it can hardly be supposed that microscopic research will
ever be able to establish the fact, and that for reasons which must now
be explained.

Let us suppose that we visit a vast botanic garden, and in the seed-time
of each of the plants therein contained select from each plant a single
ripe seed. It is clear that, if we take home that collection of seeds,
we shall have in them a miniature picture of the garden from which they
were culled, or at least we shall be in possession of the potentiality
of such a garden, for, if we sow these seeds and have the good fortune
to see them all develop, take root and grow, we shall actually possess a
replica of the garden from which they came. Not exactly, it may be
urged, for the distribution or arrangement of the seeds mus