No. 507] DEVELOPMENT AND HEREDITY 



177 



fourth-year inbred families was found to be heterozygous 

 in the character pair presence and absence of color in 

 the silk. But there is absolutely no indication that there 

 is loss of vigor after the isolation of a homozygous in- 

 dividual. 



It seems, then, that this type of degeneration (the 

 common type) is limited to a partial loss of power of 

 development, a reduction in rapidity and amount of cell 

 division. The phenomenon is readily apparent in open 

 fertilized plants like maize, for there the vigorous grow- 

 ing hybrids are continually being formed in nature. 

 When the components of these hybrid strains are isolated 

 by inbreeding, reduction in vigor is immediately seen. 

 In plants like tobacco, which are naturally inbred, no 

 degeneration is suspected, for the natural plants are 

 taken as the standard. There is an increase in vigor, 

 however, when inbred tobacco strains are crossed, and if 

 the F l generation is then taken as a standard, there is a 

 loss of vigor through inbreeding comparable to that 

 which takes place in maize. 



Upon what theoretical basis can these facts rest? In 

 the first place, whether or not we accept the theory that 

 the nucleus is the bearer of all hereditary characters, 

 nevertheless we must believe that amphimixis has two 

 functions, the one a recombination of hereditary char- 

 acters, and the other a stimulation to development. If 

 we postulate that there is an increase in this stimulation 

 when two strains differing in gametic structure are com- 

 bined, we satisfy all observed conditions. This will ex- 

 plain why decrease in vigor and not degeneration of char- 

 acters is usually the sole effect of inbreeding, and will 

 also show why this decrease must reach a limit with the 

 complete isolation of an individual homozygous in all 

 characters, and never will result in a complete degenera- 

 tion, or < 'running out," of the strain. One other effect 

 sometimes noticed in inbreeding is also thoroughly in 

 accord with the hypothesis. This is decrease in fertility. 

 Since fertility must necessarily start with the union of 



