44: EVOLUTION OF LIVING ORGANISMS. 



CHAPTER IV 



VARIATION AND THE FACTORS OF INHERITANCE : 

 DETERMINATION OF SEX 



LET us return again to the consideration of the variation 

 universally shown among all living organisms. Within 

 the range of modifiability of a given organism the modi- 

 fication may vary in amount according to the duration 

 or intensity of the stimulus. Could we obtain a large 

 number of creatures all endowed with the same inherit- 

 ance, and grow them under perfectly uniform conditions, 

 they would give rise to exactly similar individuals 

 there would be no variation. Grown on the contrary 

 under variable conditions, the resulting diversity would 

 give us a measure of the modification. In the same way 

 with a number of individuals grown under exactly the 

 same conditions, the resulting variation would give us a 

 measure of the hereditary differences. Such simple con- 

 ditions are never quite realised in nature or in experi- 

 ments ; but we can get approximations. 



Take, for instance, the case of the beans worked out by 

 Johannsen. 



The flowers of the common bean, Phaseolus vulgaris, 

 fertilise themselves ; all the offspring of one bean, then, 

 may have approximately the same hereditary factors. 

 Since the weight of the bean-seeds is affected by various 

 independent environmental factors, such as number and 

 position of the beans in the pod, richness of the soil, light 

 and shade, and so forth, and the particular combinations 

 of favourable and unfavourable stimuli fall haphazard on 

 each particular bean, the variation in weight of the beano 

 of each plant will be found to follow the normal curve of 

 probability (p. 34). The mean weight will be the most 

 frequent, and round it the modifications will fluctuate 

 in decreasing number towards the two extremes. Now, 



