HEREDITY IN PLANTS, ANIMALS, AND MAN. 300 
name of Lamarck, that variations were brought about by the direct 
action of the environment——of the conditions under which the life of the 
animal or plant was carried on—and that variations originating in this 
way were capable of being inherited. Changes of structure brought about 
by the use or disuse of organs were considered of particular importance 
in this connection. Especially when the environmental conditions had 
remained the same for many generations did the characters produced by 
them become, it was thought, permanently fixed in the species as part of 
its hereditary constitution. This view was strongly attacked by Weis- 
mann, who held that variations produced by external conditions, and 
especially by use and disuse of parts, were not hereditary, and the char- 
acters of the germ plasm as received from the parents were transmitted to 
the offspring without change. Weismann’s view has been considered the 
more probable by the majority of biologists since he wrote, though by a 
minority it has always been subjected to vigorous criticism. 
With the question of the causes that give rise to variations capable of 
being inherited, it is not however my intention to deal to-night. Accept 
ing the fact, which cannot be disputed, that such variations do occur, and 
recognising that characters of the parent sometimes do and sometimes 
do not appear in the offspring, when parents with different characters 
are mated together, we shall consider the system or law in accordance 
with which the hereditary transmission of characters takes place. 
What is now recognised as the epoch-making pioneer work in this 
subject was carried out by Gregor Mendel, Abbot of Briinn, a small town 
in Austria, and published by him in the Proceedings of the local natural 
history society at Briinn in 1865. This work unfortunately escaped 
attention for many years, and it was not until 1900 that the importance 
of Mendel’s paper was recognised by de Vries, Correns and Tschermak, 
who all three about the same time brought it into notice. The work has 
since been repeated and extended by Bateson, Punnett and many other 
workers in this country, on the Continent, and in America, and to-day 
there is a very extensive literature dealing with heredity on Mendelian 
principles in plants, animals, and man. 
Mendel chose the common garden pea for the purposes of his experi- 
ments, a plant which exists in a number of well-marked varieties, capable 
of being crossed easily one with the other. The nature of his experiments 
and the character of the results which he obtained will perhaps be made 
clear by the description of a simple example. There is one variety of the 
pea plant that produces seeds, which when the pods are fully ripe and 
dry are of a uniform yellow colour. Another variety, on the other hand, 
produces peas which when they are ripe and dry are green. 
It will be well known to you that in order to produce a ripe seed, which 
