70 BASIS OF THE PKOBLEM 



in the stimuli which play upon them. Obviously the stimuli do 

 differ ; the weight is affected by differences in light, shade, 

 number, and position of the beans in a pod and so on. But since 

 the offspring of a single plant have approximately the same 

 germinal constitution, the average weight of the offspring of light 

 beans, which are light because of subjection to less favourable 

 stimuli than the average, will not be on that account lighter than 

 the mean weight of the strain, and similarly mutatis mutandis 

 with regard to the offspring of the heavy beans. 



It follows, therefore, with regard to this particular character 

 that there are a number of different strains in the population. 

 The beans arising from any one strain are not of the same weight 

 because they have been subjected to different stimuli. If each 

 strain was pure and if differences in environmental stimuli could 

 be removed, then beans gathered from a bean-field would not 

 exhibit a simple gradation in weight ; there would be a number 

 of steps and as many steps as there were strains. In actual fact, 

 the modifications produce the gradation which is observed. The 

 fact that the nature of the germinal constitution cannot, as stated 

 above, be determined by a mere inspection of the characters is 

 exemphfied by this experiment. If we take a bean weighing, 

 say, 55 centigrams, it may belong to a strain which varies, say, 

 between 20 and 65 centigrams, or to a strain which varies between 

 40 and 90 centigrams. It is only when this bean is sown, and the 

 weight of the offspring calculated, that the strain to which it 

 belongs can be ascertained. The experiment further shows why 

 regression towards the mean is observed, when, instead of limiting 

 investigation to a ' pure line ', beans differing in a certain respect 

 from the average, by, for instance, greater weight, are selected 

 and sown. Such beans will belong to several different strains ; 

 they will, however, include more which have been favourably 

 than unfavourably influenced by the surroundings. The average 

 of the weight of all their offspring will therefore be less than the 

 average weight of the parents. At this point we touch upon 

 the problem of selection, but before we go on to deal with this 

 question, it is necessary to say something more about variation 

 and the origin of variations. 



5. Self-fertilization is very exceptional, and what happens in 

 the case of the beans, though illuminating, is not typical. As 

 a general rule in reproduction, two parents contribute to the 



