THE INDIVIDUAL AND THE SPECIES 



189 



the environment of an organism may be extended 

 so as to include the medium in which embryonic 

 development took place, and even the parental body 

 which formed the environment for the germ-cells 

 from which embryonic development began. But it 

 is probably the case that even with an uniform en- 

 vironment, or with one in which the differences 

 were insignificant, variability would still exist. The 

 variations that might be observed in such a case 

 would belong to two kinds — " fluctuating variations," 

 and " mutations." 



4 



J>Jean 



2- 4-- 



Mean. 



H 



Ylholt 



po/ialaTiOri 



~+ 



F>rsT ,. 



generation 



Fig. 21. 



z4Mea.n1 



Seco riot 

 generation . 



Whether the variations observed in a population 

 of organisms are fluctuations or mutations can only be 

 determined by experiment. Let us suppose that we 

 are dealing with a human population, and that the 

 variation studied is that of stature. Let the men 

 with statures considerably over the mean value marry 

 the women who are correspondingly tall, then it will 

 be found that the children from these unions will, 

 when grown up, exhibit a stature which is greater 

 than that of the whole population, but not so great 

 as that of their parents — that is, regression towards 

 the mean of the whole population takes place. 



This is shown in the above diagram, where the 



