e unexpected by Darwin and his contemporaries.
But it is polymorphic variability in the strictest sense of the word.
How the single constituents of a variety originate we do not see. We
may assume, and there can hardly be a doubt about the truth of the
assumption, that a new character, once produced, will slowly but surely
be combined through accidental crosses with a large number of
previously existing types, and so will tend to double the number of the
constituents of the variety. But whether it first appears suddenly or
whether it is only slowly evolved we cannot determine. It would, of
course, be impossible to observe either process in such a mixture. Only
cultures of pure races, of single-parent races as we have called them,
can afford an opportunity for this kind of observation. In the fields of
Svalof new and unexpected qualities have recently been seen, from time
to time, to appear suddenly. These characters are as distinct as the
older ones and appear to be constant from the moment of their origin.

Darwin has repeatedly insisted that man does not cause variability. He
simply selects the variations given to him by the hand of nature. He may
repeat this process in order to accumulate different new characters
in the same family, thus producing varieties of a higher order. This
process of accumulation would, if continued for a longer time, lead to
the augmentation of the slight differences characteristic of varieties
into the greater differences characteristic of species and genera. It is
in this way that horticultural and agricultural experience contribute
to the problem of the conversion of varieties into species, and to the
explanation of the admirable adaptations of each organism to its complex
conditions of life. In the long run new forms, distinguished from their
allies by quite a number of new characters, would, by the extermination
of the older intermediates, become distinct species.

Thus we see that the theory of the origin of species by means of natural
selection is quite independent of the question, how the variations to
be selected arise. They may arise slowly, from simple fluctuations, or
suddenly, by mutations; in both cases natural selection will take hold
of them, will multiply them if they are beneficial, and in the course of
time accumulate them, so as to produce that great diversity of organic
life, which we so highly admire.

Darwin has left the decision of this difficult and obviously subord