inbreeding, practically bred true, provided they were always sub- 
jected to the same environmental conditions. Certain varieties of 
wheat thus treated have remained the same for forty years, and 
the Vilmorin seed firm of Paris has a series of excellently 
preserved herbarium specimens whereby this statement may be 
proved. On the basis of this and similar evidence, selection is 
not a creative agent. Selection, so many of us believe, cannot 
change the nature of the hereditary units. One may never hope 
to secure blue roses from red roses by selection. One must 
patiently wait for the variation to arise, caused by no one knows 
as yet what, and then isolate it. Very often variations that arise 
in this manner breed true at once. 
Inasmuch as most of the higher plants and animals result 
from a union of two microscopic cells, and as each of these cells 
may carry the hereditary units which together with environment, 
are responsible for the characters of these organisms, it follows 
that every sexually produced animal or plant that breeds true for 
several successive generations must have at least two of these 
units, identical in every respect, for each of the characters in which 
it breeds true. The only exceptions are those cases where a num- 
ber of characters result from one kind of unit, in which case two 
identical hereditary units may be sufficient for several char- 
acters. 
Granting then, that the Mendelian method of analysis is ap- 
plicable to practically all the characters of the world of living 
organisms, — that animals and plants are made up of independently 
inherited units which, in contact with environment, express them- 
selves as characters, and granting that these units are unvari- 
able and not modifiable by selection, how can we account for all 
the variability we see about us ? 
If our modern conceptions of the composition of organisms 
hold, variation may only occur: 
1. When the complex of hereditary units constituting the organism 
remain the same, and the external environment (e.g , soil, climate, food.) 
is changed. 
2. When one or more of these units is added or subtracted from the 
organism (through crossing), the external environment remaining 
unchanged. 
3. When both the complex of hereditary units and the environment 
are altered. 
4. When both the complex of hereditary units and the environment 
remain the same, but a new hereditary unit is added or subtracted 
(mutation). The method by which this is accomplished is unknown. 
All four of these statements are illustrated by the experiments 
in progress in the plant -breeding section of the Brooklyn 
Botanic Garden. However, all except the last one of them 
