196 OUTLINES OF EVOLUTIONARY BIOLOGY 



successive generations. It was the discovery of these proportions 

 that furnished the clue to the mystery. 



Mendel found the material which he required for his experi- 

 ments chiefly in the numerous garden varieties of the common 

 edible pea. How these varieties first originated we do not know. 

 Possibly they arose as sudden and apparently spontaneous 

 variations, or mutations. No less than twenty-two such varieties 

 were chosen for experiment. 



The next thing was to select certain differentiating characters 

 upon which to concentrate attention, and this is a very important 

 point. Of course, all the varieties agreed with one another in 

 most of their characters ; in other words, they were all edible 

 peas exhibiting the specific characters of the plant known to 

 botanists as Pisum sativum, but they differed in numerous minor 

 features. Of these differentiating characters Mendel selected 

 seven for the purposes of his experiments, amongst which I need 

 mention only two : (1) the form of the ripe seed, which, roughly 

 speaking, may be either round and smooth or angular and 

 wrinkled ; and (2) the difference in colour of the seed-contents or 

 cotyledons, which may be either yellow or green and which usually 

 determine the colour of the seed as a whole. 



The two differentiating characters of each pair were artificially 

 united by cross-fertilization, the flowers of a round-seeded pea 

 being fertilized by the pollen of a wrinkled-seeded pea, those of a 

 green-seeded pea by the pollen of a yellow-seeded pea, and so on. 



In this way a number of hybrid plants, represented in the first 

 instance by seeds, were obtained. 1 These hybrids constituted 

 what modern writers term the F l or "first filial" generation, 

 and the curious fact was observed that every hybrid closely 

 resembled one of the two parents, instead of being, as is frequently 

 the case in other hybrids, intermediate in character between 

 them. Further experiments with the offspring of the hybrids, 

 however, showed that, although only one of the two contrasted 

 characters manifested itself in the hybrid, the other was there 

 also in a latent or dormant condition, and appeared again in 



1 It must be remembered that the pea-seed consists chiefly of the young plant 

 in a dormant condition, and that its characters are therefore, with the exception 

 of those appertaining to the seed-coat, which are not considered in this 

 chapter, those of the sporophyte generation which follows the conjugation of the 

 gametes. The fact that the complete life-cycle of the plant really includes two 

 generations, a well developed sporophyte and a vestigial gametophyte, does not 

 affect the Mendelian conclusions. 



