HEREDITY 



503 



as social and other somewhat artificial conditions of life may 

 intervene, and we shall anxiously await the results of further 

 investigation into the character and range of difference between 

 offspring of the same parents. 



Individuality. Whatever the results of investigation in this 

 direction, and whatever gradual advance or decline may be 

 established among the members of the same family on the aver- 

 age, the fact remains that variability is largely heterogeneous as 

 between individuals, and that a marked individuality pervades 

 all offspring, either of the same or of different parents. 



This lessened deviation between members of the same family 

 as compared with descendants in general is due to the fact that, 

 among brothers, not only both immediate parents but all ances- 

 tors are identical. The differences that do exist within the same 

 family serve to show the wide divergencies possible with the same 

 hereditary elements, although, in studying adults, some allow- 

 ances must always be made for differences in development due 

 to external causes. While members of the same family are in gen- 

 eral reared more nearly alike than are members of different fam- 

 ilies, yet in a large sense every individual has a life history of his 

 own quite distinct and in many senses different from that of any 

 other individual of his own or of any other generation. Now this 

 life history affects development and accounts for some of the 

 differences between adult individuals. Not all of the variability 

 within a family can, therefore, be assigned to hereditary influ- 

 ences, but that a large share of it is due to such influences is 

 rendered extremely likely by the well-known fact that successive 

 ova, spermatozoa, or pollen grains from the same individuals 

 are not alike either in their genesis or in their behavior after- 

 ward. The very mechanism of maturation strongly suggests 

 profound qualitative differences, and tends to modify our as- 

 sumption that all children of the same parents possess identical 

 hereditary elements. The experience of breeders everywhere is 

 that offspring of the same individuals are not slightly different 

 but in general they are widely different. Whether, and to what 

 extent, these differences can be lessened by selection and by 

 relative purity of ancestral gametes are questions on which light 

 is sorely needed. 



