cannot see
how alterations in the structure of mature body parts through use and
adjustment to the environment can be introduced into it to become new
qualities of the species.

It must be clear, I am sure, that this theory supplements natural
selection, for it describes the physical basis of inheritance, it
demonstrates the efficiency of congenital or germ-plasmal factors of
variation in contrast with the Lamarckian factors, and finally in the way
that in the view of Weismann it accounts for the origin of variations as
the result of the commingling of two differing parental streams of germ
plasm.

At first, for many reasons, Weismann's theories did not meet with general
acceptance, but during recent years there has been a marked return to many
of his positions, mainly as the result of further cytological discoveries,
and of the formulation of Mendel's Law and of De Vries's mutation theory.
The first-named law was propounded by Gregor Mendel on the basis of
extensive experiments upon plants conducted during many years, 1860 and
later, in the obscurity of his monastery garden at Altbruenn, in Austria.
It was rescued from oblivion by De Vries, who found it buried in a mass of
literature and brought it to light when he published his renowned Mutation
Theory in 1901. Mendelian phenomena of inheritance, confirmed and extended
by numerous workers with plants and animals, prove that in many cases
portions of the streams of germ plasm that combine to form the hereditary
content of organisms may retain their individuality during embryonic and
later development, and that they may emerge in their original purity when
the germ-cells destined to form a later generation undergo the preparatory
processes of maturation. They demonstrate also the apparent chance nature
of the phenomena of inheritance. To my mind the most striking and
significant result in this field is the demonstration that a particular
chromosome or chromatin mass determines a particular character of an adult
organism, which is quite a different matter from the reference of all the
hereditary characters to the chromatin as a whole. Wilson and others have
brought forward convincing proof that the complex character of sex in
insects actually resides in or is determined by particular and definite
masses of this wonderful physical basis of inheritance.

Mendel's principles also account in the most remarkable way for many
previously obscure phenomena, like reversion, or