762 
of such pairing of unrelated characters are 
known in both plants and animals. 
About 1903 a new conception of the pair 
of allelomorphs arose. In that year, both 
Correns and Quenot foreshadowed what 
was more definitely suggested by Bateson 
and Punnett in 1905, as the “‘presence and 
absence hypothesis.”? C. C. Hurst the 
next year developed this hypothesis to its 
logical conelusion.® 
According to this hypothesis the differ- 
ence, say, between two varieties of cow- 
peas, one of which has red and the other 
white seed coats, is due to the presence of 
an allelomorph for red in the one case and 
its absence in the other. When the re- 
ducing division occurs in the hybrid this 
allelomorph was supposed to pass entire 
into one of the daughter cells, thus giving 
two types of gametes, one with, the other 
without, the potentiality of producing red 
pigment in the seed coat. It is usually 
assumed that the dominant member of the 
character pair corresponds to the presence, 
and the recessive member to the absence, of 
an allelomorph, though there are apparent 
exceptions to this rule. 
While some biologists still adhere to the 
idea that the organism may be regarded as 
a structure built up of parts each of which 
represents a separately inherited character 
and is represented in the germ plasm by a 
pangene, this idea is much less prevalent 
than it was a few years ago. Recently the 
attempt has been made to formulate a 
description of Mendelian phenomena in 
terms that do not involve any hypothesis 
concerning the nature and interaction of 
the germ plasm elements which are cer- 
tainly responsible for these phenomena. 
Notable amongst such attempts is the 
““venotype-conception’’ of Johannsen. 
*T am also indebted to Dr. Shull for these his- 
torical facts concerning the presence and absence 
hypothesis. 
SCIENCE 
[N.S. Vou. XXXV. No. 907 
Johannsen does not define very definitely 
his term ‘‘gene,’’ further than to apply it 
to the cell organs or cell substances, what- 
ever they may be, that are responsible for 
Mendelian phenomena. He particularly 
insists that the gene is not to be regarded 
as the basis in the germ plasm of a par- 
ticular character, but that Mendelian phe- 
nomena arise from differences in corre- 
sponding genes in two varieties. This idea 
is in close accord with that of Mendel, 
which we have already discussed. Johann- 
sen’s gene and Mendel’s formative element 
appear to be the same thing. Any par- 
ticular character is probably the result of 
the interaction of several or many genes, 
and any one gene may bear a relation to 
many characters. 
In a field of investigation where so much 
confusion has existed as to the meaning of 
terms, and where widely different views 
have been maintained as to the significance 
of the phenomena observed, it is necessary, 
in order to render discussion fully intelli- 
gible, to describe quite accurately the facts, 
which are not in dispute, and thus attach 
definite meanings to the terms used. It is 
not so important that we should agree as to 
the proper use of terms as that we should 
understand clearly the actual meaning of 
a writer, whether he uses terms correctly or 
not. In order that the meaning which I 
attach to the term ‘‘ Mendelian factor’’ may 
be made clear, I shall set forth in some 
detail the facts about a group of these fac- 
tors with which I have been working for 
some years past. 
In my investigations of heredity in the 
cowpea (Vigna wngwiculata) the seed-coat 
colors found are as follows: white (or 
cream), red, buff, blue, brown, black and 
purple. In addition to the simple colors 
above enumerated, certain varieties have 
the surface more or less thickly covered 
with bluish purple dots (speckling). In 
