and physicists. In this sense, a character must always be looked 
upon as the combined result of heredity and environment. The 
heredity units themselves are not characters in any sense, but, in 
conjunction with environment, express themselves as characters. 
The units themselves are to be regarded as the primary elements 
of the “living” world, just as oxygen and sulphur represent ele- 
ments of the so-called inorganic world. Another modification of 
Mendel's conception is that of linkage. For we find that certain 
characters are either partially or always inherited along with other 
characters, not always separately as Mendel supposed. Many 
of these characters in certain animals are sex-linked. In the little 
banana fly, over one hundred of these differential heredity units 
have been studied and instead of finding that each is inherited 
independently of all the others, the discovery has been made that 
they are inherited as though more or less loosely linked together 
in tour groups. In plants similar discoveries have been made 
though not on as large a scale. In peas themselves, colored 
flowers are found to be partially linked or coupled with the mater- 
ials responsible for late flowering, while the white flowers are 
partially associated with thematerials responsible for early flower- 
ing time. One pea variety with no tendrils and having wrinkled 
seed, crossed with a smooth seeded, tendrilled variety gives only 
smooth seeded, tendrilled offspring in the first generation, while 
in the second, where four types of plants are expected in the pro- 
portion of 9ST: 3WT: 3SN — T: 1WN — T, approximately only two 
appear, the two centralgroups being represented by very few indi- 
viduals. The two outside groups of course are the same combina- 
tions as found in the original parents of the cross, and though the 
two characters naturally would not be expected to be associated in 
any way, experimental work has shown us that they are almost 
completely coupled. There are numerous other modifications of 
Mendel’s ideas, but all of these, though important, are to be re- 
garded as superstructures built bv the methods and on the foun- 
dation stones laid down by Mendel. 
Through the light thrown upon the nature and inheritance of 
characters by Mendelism, experimenters soon came to a radically 
different conception regarding the role selection played in the 
creation of new varieties. They found that certain varieties of 
wheat, beans, and other plants, once freed from impurities by 
constant inbreeding, practically bred true, provided they were 
always subjected to the same environmental conditions. Certain 
varieties of wheat thus treated by the Vilmorin family of France 
have remained stable for forty years. Some varieties of peas are 
said to have bred true for a still longer period. Sugar per cent 
in sugar beets has not been appreciably increased over that ob- 
tained during the first few years when the selection for a high 
percentage of sugar began, although more stable varieties have 
been obtained. On the basis of this kind of evidence, together 
with that obtained from more recent experimental studies, selec- 
tion is found not to be the creative agent Darwin and most of the 
eighteenth century breeders and scientists supposed. Selection, 
so many experimenters believe, cannot alter the nature of the 
hereditary units, nor can it cause them to come into existence. 
Chance, or fortuitous variation, brought about no one knows 
how, seems to be the only method by which new units arise. To 
obtain a blue rose from a red-flowered variety, one must wait, so 
most scientific breeders believe, until a blue-flowered variation 
occurs — for selection is of no avail except as a method of isolating 
desired variations that are already in existence. When such 
variations arise in naturally inbred plants, such as peas, they 
generally breed true at once; but in cross-fertilized species, such 
as corn, purification by selection must be practiced to secure a 
true breeding strain. 
