ent, (2) _not_ in the main to the activities of the animal
through the use or disuse of its parts, (3) _not_ to any innate principle
of living material itself, and (4) above all _not_ to purpose either from
within or from without. Darwin made quite clear what he meant by chance. By
chance he did not mean that the variations were not causal. On the contrary
he taught that in Science we mean by chance only that the particular
combination of causes that bring about a variation are not known. They are
accidents, it is true, but they are causal accidents.

In his famous book on "Animals and Plants under Domestication", Darwin
dwells at great length on the nature of the conditions that bring about
variations. If his views seem to us today at times vague, at times
problematical, and often without a secure basis, nevertheless we find in
every instance, that Darwin was searching for the _physical causes of
variation_. He brought, in consequence, conviction to many minds that there
are abundant indications, even if certain proof is lacking, that the causes
of variation are to be found in natural processes.

Today the belief that evolution takes place by means of natural processes
is generally accepted. It does not seem probable that we shall ever again
have to renew the old contest between evolution and special creation.

But this is not enough. We can never remain satisfied with a negative
conclusion of this kind. We must find out what natural causes bring about
variations in animals and plants; and we must also find out what kinds of
variations are inherited, and how they are inherited. If the circumstantial
evidence for organic evolution, furnished by comparative anatomy,
embryology and paleontology is cogent, we should be able to observe
evolution going on at the present time, i.e. we should be able to observe
the occurrence of variations and their transmission. This has actually been
done by the geneticist in the study of mutations and Mendelian heredity, as
the succeeding lectures will show.

* * * * *

CHAPTER II

THE BEARING OF MENDEL'S DISCOVERY ON THE ORIGIN OF HEREDITARY CHARACTERS

Between the years 1857 and 1868 Gregor Mendel, Augustinian monk, studied
the heredity of certain characters of the common edible pea, in the garden
of the monastery at Bruenn.

In his account of his work written in 1868, he said:

"It requires indeed some courage to undertake a labor of such a