326. T. H. Morgan 
for, in several insects where the number of chromosomes is small 
the pairs have been closely studied by Wilson and no differences 
discovered. 
In 1906, on the basis of his own observations relating to the 
number of chromosomes in the male and female and of the similar 
results of Stevens, Wilson discussed very fully two alternative 
views of sex determination. ‘The first rested on the assumption 
of male-determining and female-determining chromosomes. He 
pointed out that this hypothesis involves the idea of selective fertili- 
zation, in favor of which Cuénot’s hypothesis for yellow mice 
is cited. Asa second, “alternative hypothesis’? Wilson suggested 
that the “differential chromosomes may perform a definite and 
special function in sex production without being themselves speci- 
fically male-determining and female-determining, or even qualita- 
tively different save in the degree of their special activity (what- 
ever be its nature). The kind of activity that produces a male 
will, if reénforced or intensified, produce a female.” 
In the case of the bee he points out the difference may depend on 
fertilization or its absence. Where an unpaired accessory exists, 
as in Anasa tristis, the presence of this one, and of another of the 
same kind (in the egg) produces a female, while in the male one 
of these alone is the cause of that sex developing. But when, as 
in other cases, there is no unpaired accessory, but a corresponding 
pair of equal sizes, the quantitative result no longer holds. ‘There- 
fore Wilson suggests that one member of the pair in the male 
may show less activity than its mate. In consequence the egg 
fertilized by such a sperm becomes a male, and the weakened 
chromosome is confined to the male line. ‘This suggestion is 
somewhat similar to the one discussed above in which the weaker 
chromosome is represented as the female determiner (F’). Ina 
later paper, to be referred to in another section, Wilson suggests 
a third view differing from both of these, but more nearly akin to 
the second one here described. 
THE BOTANICAL EXPERIMENTAL EVIDENCE 
The important experiments of Blakeslee, of the Marchals and 
of Correns on plants have an important bearing on the problem of 
