28 The Origin of a Mutation i7i the Siveet Pea 



cleft keel (cf. Fig. 1). The standard and wings are generally smaller 

 than in the normal flower and fail to expand fully, but in these respects 

 a good deal of variation is to be found. The cretin is however always 

 characterised by one other feature ; it is invariably sterile on the female 

 side. The fact that this peculiar form appeared as a single individual 

 in a pedigree culture has already been recorded. Data have also been 

 given to shew that it behaves as a simple recessive to the normal form^ 

 There arises the question whether the evidence is consonant with the 

 view that the original mutation occurred in the maturation divisions of 

 the germ cells, or at some other stage. To attempt to answer it involves 

 a consideration of all the details connected with the coming of the 



Fig. 1. Two flowers of the original cretin plant, No. 14618/1907. 

 Flowers of other cretins are figured on PI. XL, Journ. Gen. 

 Vol. I. 1911. 



cretin, in so far as they are known, together with those of its subsequent 

 behaviour. 



In 1903 a cross was made between two white sweet peas, Blanche 

 Burpee (long pollen) and Emily Henderson (round pollen)^ From 3 

 purple F^ plants three large F^ families were raised in 1905. From one 

 of these F^ families, No. 309 containing 187 plants, the seed of 29 in- 

 dividuals was saved to give an F.;. generation. These 29 families were 

 raised in 1906 and resulted in 2083 individuals all of which were normal 



1 Journal of Genetics, Vol. i, 1911 ; ibid. vol. iii. 1913. 



^ Cf. Bep. Evol. Gomm. Roy. Soc. iv. 1908, pp. 9 seq. where details will be found of the 

 various characters entering into the cross. 



