36 GENETICS 



causes of variation, at least of heritable variations, 

 are intrinsic or inborn in the germplasm. His con- 

 ception of sexual reproduction is that it is a device for 

 doubling the possible variations in the offspring by 

 the mingling of two strains of germplasm (amphimixis). 

 By far the greater number of observations recorded 

 go to substantiate this theory. 



A 



Number of 

 Flowers 

 80 1 



70 



10 98765 10 9876543 10 987654 



Number of Stamens 



FIG. 10. Variations in the number of stamens in the flowers of 

 the "live-for-ever" (Sedum spectabile) under various con- 

 trolled conditions. For detailed description, see text. After 

 Klebs. 



Tower found among his potato-beetles, for example, 

 that two strains reared in the same environment showed 

 striking differences in variation, a fact necessarily due 

 to intrinsic rather than to extrinsic factors. Similar 

 cases may be recalled by any one. 



Nevertheless, heritable variation occurs in the ab- 

 sence of amphimixis so that, at best, sexual reproduc- 

 tion furnishes only one of the possible avenues for the 

 introduction of hereditary variations. 



