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 number of characters result from one kind of unit, in which 
case two of these units, exactly alike, maybe responsible for 
several characters, as, for example, in the characters pink flower 
color, red leaf axil color, and gray seed coat color, in peas. 
Granting for the time being that the Mendelian method of 
analysis is applicable to practically all the characters of the 
world of living organisms, — that plants and animals are made up 
of independently inherited units, which, in combination with en- 
vironment and with each other, express themselves as characters; 
and granting further that these units are unvariable and not 
modifiable by selections, how can we arrange and account for all 
the variation we see about us ? 
If the conceptions ot Mendel and his followers hold as regards 
the composition of organisms, variations may only occur: — 
1. When the complex of heredity units constituting 
the organism remains the same, and the external environ- 
ment (e. g., soil, climate, food) is changed. 
2. When one or more of these units is added to or 
subtracted from the organism (through crossing), the ex- 
ternal environment remaining unchanged. 
3. When both the complex of hereditary units and the 
environment are altered. 
4. When the environment remains the same, but the 
complex of hereditary units (the organism) is altered by 
the “addition” of a new hereditary unit, or the “subtrac- 
tion” of an old one (mutation). 
All four of these statements find illustration in almost every 
plant breeder’s experimental garden. In the Brooklyn Botanic 
Garden, the plant material for the study of heredity is largely 
made up of peas and castor beans, in which many units have 
been isolated. In Boston, at the Bussey Institution of Harvard 
University, the material is tobacco and corn; and among animals, 
mice, guinea pigs, rats, and rabbits are undergoing investiga- 
tion. At Cornell University, beans and garden phlox are being 
studied; while at Columbia University students are working on 
thousands of the little banana fly, in over one hundred and fifty 
varieties. In peas, about thirty of these units have been isolated, 
so that some varieties of peas written in Mendelian formula 
would appear as AABBCCDDEEFFGG, etc., each letter standing 
for a particular type of unit. In corn, as many as thirty are 
known, while in the banana fly about a hundred have been 
isolated. 
In a former Leaflet (Ser. Ill, No. 10 & 11), directions are 
given for carrying out plant-breeding experiments in one’s own 
garden — a field capable of affording much pleasure and some 
profit. Orland E. White. 
Current numbers of Leaflets are free to all who wish them. 
Back series, complete, 50c. each; single numbers, 5c. each. 
The Leaflets are published weekly or bi-weekly from April to June, and 
September to October, inclusive, by The Brooklyn Institute of Arts and Sciences, 
at Washington Avenue and Montgomery Street, Brooklyn, N. Y. 
Telephone: 6173 Prospect. 
Mail address: Brooklyn Botanic Garden, Brooklyn. N. Y. 
