D. J. SCOURFIELD ON MENDELISM AND MICROSCOPY. 397 



protected, either naturally, or at least by easy artificial means, 

 from extraneous pollen ; and further, that the hybrids must not 

 show any serious diminution in fertility. All these desirable 

 qualities he found, after some preliminary investigations, to be 

 offered by varieties of the garden pea, Pimm sativum, and it was 

 with these that most of Mendel's work was done. The carrying 

 out of the second condition mentioned — namely, the recording 

 of the results obtained from every separate seed for several 

 generations — involved, of course, an enormous amount of labour • 

 but it proved to be a step of the greatest value, and was abun- 

 dantly justified by the result, for it enabled Mendel to get, from 

 the progeny, an insight into the constitution of the germ-plasm of 

 the parents in a way which had never been done before. Even 

 more important, perhaps, was the third condition, for the limita- 

 tion of the observations to a few well-marked characters not only 

 brought the task within something approaching manageable 

 proportions, but it constituted a method of analysis which marked 

 a new era in the experimental study of heredity, and it led 

 Mendel directly to the formulation of the very definite law which 

 now bears his name. 



Confining our attention for the present to Mendel's experiments 

 with peas, we may note that he selected for observation seven 

 contrasted pairs of characters, namely, — 



1. Form of the ripe seeds, whether round or wrinkled; 



2. Colour of the cotyledons, whether yellow or green ; 



3. Colour of the seed-coat, whether grey or white ; 



4. Form of the ripe pod, whether inflated or constricted ; 



5. Colour of the unripe pod, whether green or yellow; 



6. Position of the flowers, whether axial or terminal ; 



7. Length of the stem, whether long (6 — 7 ft.) or short (f — 1 h ft.). 

 Plants belonging to varieties possessing one of the characters of 



one or more of the above pairs were united by cross-fertilisation 

 with plants exhibiting the other character, and reciprocal crosses 

 were made in all cases — e.g. the stigmas of "round pea" flowers 

 would be dusted with pollen from '•'wrinkled pea "flowers, and 

 vice versa. 



In every case Mendel found that in the hybrids produced from 

 these first artificial crosses, one character of each pair appeared to 

 the total exclusion of the other, and this was true whichever 

 way the cross was made. The character thus entirely overpower- 



