316 LECTURES ON BIOLOGY 



to publish his results. We shall here consider only some of the 

 most important results at the hands of a few simple instances, 

 and follow not only the experiments of Mendel but also those of 

 more recent investigators. 



The case is simplest when both parents used for crossing 

 differ in only one point. In that case we are dealing with two 

 characters, one from the father and one from the mother. Both 

 together are described as 'character-pair.' If, for instance, 

 we cross a red-flowering and a white-flowering pea the colours 

 'red' and 'white' forms the 'character-pair.' The bastards or 

 hybrids which develop from such cross are all red-flowering ; they 

 all follow one of the parents and cannot be distinguished from 

 them. The white colour seems to be entirely suppressed. But if 

 we breed from these red-flowering hybrids and fertilize them with 

 their own pollen we see a remarkable phenomenon. Though in 

 this case the ' father ' as well as the ' mother ' had red flowers 

 the second hybrid generation produces only 75 per cent, of red- 

 flowering descendants, while in the rest the white flower of 

 the one 'parent,' which in the first generation had entirely 

 disappeared in favour of red, reappears here suddenly anew. 

 If the breeding is continued we see that the white-flowering 

 hybrid form under self-fertilization remains from now perma- 

 nently constant, producing always white-flowering descendants. 

 In the red-flowering variety, however, we find that under equal 

 conditions in all subsequent generations always the same division 

 takes place, i.e., we always obtain 75 per cent, red-flowering 

 ' dividing ' and 25 per cent, white-flowering ' constant ' peas. 



The same regularity may be observed in crossing two species 

 of nettle, of which one, Urtica patulifera, has dentate, the other, 

 Urtica dodartii, entire leaves. Here in the first generation the 

 ' character-pair ' of the parents, dentate and entire leaf, produces 

 exclusively bastards with dentate leaves, indistinguishable from 

 those of the one ' parent.' In the second generation the ' split ' 

 takes place, only three-quarters of all descendants having a 

 dentate leaf, while in the rest the suppressed character, the 

 entire leaf, reappears. The further course is identical with the 

 behaviour of the two varieties of peas. From this and other 

 facts Mendel concluded that for each independently inheritable 

 character of the parents there must be present in their idio- 



