Mutations and Evolution. 78 
keel, and it is invariably sterile on the female side. It behaves in 
crosses with the parent as a simple Mendelian recessive, a total of 
80 families in six years giving 4198 normals : 1322 cretins, which is 
not far removed from a 3: 1 ratio, the viability of the cretin being 
evidently somewhat less than that of the normal form. 
The cretin appeared in a cross between two white sweet peas, 
Blanche Burpee with long pollen and Emily Henderson with round 
pollen. From three purple F t plants large F 2 families were raised 
in 1905. From one of these F 2 families containing 187 plants, the 
seed of 29 individuals was saved for the F 3 generation. These 29 
families in 1906 resulted in 2083 plants, all normal. Seeds for the 
F 4 generation were saved from 14 individuals of one of the F 3 
families numbering 181 plants. The F 4 from these 14 plants gave 
1118 plants, one of which was the cretin. This appeared in a 
family of 52 plants raisetf from the F 3 plant 304 6 /1906. Thirteen 
sister plants yielded no cretin in a progeny of over 1000, nor did 
one appear among the 2083 plants of the F 3 generation. The P 6 
of 195 plants grown in 1912 from 9 F 4 plants, also failed to produce 
one, so it remains an isolated case. 
Punnett points out that under these circumstances it is 
unlikely that the cretin originated from two germ cells, each of 
which had lost the normal factor, for in that case the parent plant 
would have been heterozygous, giving 25% cretins, and they would 
also have appeared in the F s or in collateral families. Hence he 
concludes, in agreement with the argument above, that the cretin 
plant must have arisen through “ some radical alteration in the 
zygote after union between two normal gametes had already taken 
place,” and that it was “ due to a change in the individual at some 
stage after fertilization whereby the factor for the normal flower 
was either dropped out or altered during the somatic divisions.” 
Since the germ cells of this plant were uniform, it is reasonable to 
suppose that the change took place in the fertilized egg itself. 
Morgan (1919), in discussing this case, suggests that the 
mutation occurred in one chromosome far enough back in the 
germ track of the parent individual to give rise to pollen and ovules 
(say in one flower) each carrying the mutated genes. But in that 
case other germ cells would have been carrying these genes, and 
these would surely have appeared later either in the heterozygous 
or homozygous condition. 
As regards Drosophila , the great number of mutants it has 
produced apparently all belong to one type, and behave in the same 
