HEREDITARY QUALITIES 17 



If a more scientific proof that men are not all born alike 

 is demanded, it can be obtained by a study of identical 

 twins. The great likeness often existing between twins is 

 not to be explained by a likeness between their past sur- 

 roundings. This we know to be the case because twins 

 who were separated soon after birth, and who lived very 

 different lives, have often remained extraordinarily alike. 

 The only satisfactory explanation which can be given of 

 the great similarity between identical twins is that they 

 sprang from one and the same germ. Whatever small 

 differences there are between what are called identical 

 twins may, no doubt, be explained by differences in their 

 past surroundings. But ordinary brothers are exposed to 

 nearly the same differences in surroundings as are twins. 

 Brothers who are not twins cannot, therefore, differ from 

 each other much more than do twins because of the effects 

 of their past surroundings. To whatever extent ordinary 

 brothers differ more than twins, to that extent some other 

 explanation has to be found for these extra differences 

 between them. And no explanation can be suggested for 

 these differences except that they are due to the differ- 

 ences which existed between the germs from which the 

 brothers sprang. We thus get a rough measure of those 

 differences existing amongst the members of any one family 

 which can be traced back to their originating germs ; differ 

 ences which may be called hereditary differences. 



Here a point may be mentioned which has puzzled 

 many persons. How is it possible for persons belonging 

 to the same family to show hereditary differences ? If 

 brothers do differ widely, does not this prove that heredity 

 counts for little ? Now the quaUties of both a father and 

 a mother are to some extent, as it were, passed on to and 

 then carried by the germ out of which their son originated. 

 But the son will pass on more of his father's qualities and 

 less of his mother's to one grandchild than to another. 

 The grandchildren will in consequence differ amongst 

 themselves. And this they will do although the whole of 

 their qualities may have been derived without change 



