47 



numerical results, which Mendel obtained, can be readily 

 explained on mathematical grounds by what we know of 

 the various combinations which are possible between egg- 

 cells and pollen-grains when differing in two' characters. 

 Of course, if the parental forms differ in several characters 

 the possible combinations are much more numerous and the 

 numerical chances of complete reversion to the parent forms 

 is much smaller. On the other hand, while we obtain a 

 large number of hybrid forms which will show reversions 

 to the original parents, we shall find some new combina- 

 tions which are pure forms and therefore breed true. These 

 we can isolate from the rest by the rejection of all strains 

 which show reversion, and thus we can obtain new and 

 permanent varieties. This no doubt has been the means 

 adopted by plant breeders in the past from practical 

 experience, but thanks to Mendel and those who have fol- 

 lowed up the path shown us by his investigations, a scien- 

 tific basis has been laid to the practice of hybridisation. 



In the course of these scientific enquiries some remark- 

 able facts have come to light. It has, for instance, been 

 discovered in Crossing a white and a yellow variety of the 

 Marvel of Peru (Mirabilis) that the hybrid produced was 

 not intermediate in character, that is, of pale yellow 

 colour, neither was it like either of the parents, but of a 

 pink colour and marked with red stripes. The white form 

 must evidently have possessed some chemical factor which 

 changed the yellow colour into red, while the character 

 producing striping, which could not show itself in the 

 white form, became visible in the hybrid by reason of the 

 coloured sap. The offspring produced by self- fertilisation 

 of this hybrid were of twelve different kinds : five with 

 different shades or striping in yellow, five corresponding 

 forms in red, and two white forms, which, though re- 

 sembling each other externally, differed in constitution as 

 could be seen from their progeny. 



A somewhat similar and equally remarkable result was 

 obtained by crossing two white Sweet Peas belonging to 

 the variety, Emily Henderson, the offspring having a 

 partly-coloured flower, red with blue keel, probably very 

 like the ancestral form. Of the two white forms evidently 

 each one contained a special factor, which combined with 

 that of the other white form caused the formation of a 

 coloured sap. The offspring of the coloured hybrid were 

 mostly coloured but in different degrees, while white forms 



