288 EUGENICS 



the same fertilized ovum. In the former case the 

 twins bear no more resemblance to one another than 

 any other ordinary pair of brothers or sisters, and they 

 are often of opposite sex ; in the latter case the twins 

 are known as ' identical/ and are always of the same 

 sex. Our present knowledge of Genetics not, of 

 course, available to Galton when he first wrote upon 

 this subject leads us to believe that such twins are 

 indeed identical, and bear precisely the same hereditary 

 endowment.* It is as though a single individual were 

 divided into two parts, and each part grew into a 

 complete person. Galton quotes numerous stories 

 of the frequent confusion between identical twins. 

 ' I have one case/ he writes, ' in which a doubt remains 

 whether the children were not changed in their bath, 

 and the presumed A is not really B, and vice versa.' 

 In the records thus collected there is excellent material 

 for discovering how far identical twins can come to 

 differ from one another when exposed to different 

 conditions, and, on the other hand, for ascertaining 

 how far distinct twins brought up under similar con- 

 ditions can come to resemble one another. Galton 

 obtained information with regard to eighty cases of 

 probably identical twins. In many of these cases the 

 twins remained closely alike in temper and character, 

 as well as in appearance, up to an advanced age. 

 When differences arose in later life, these were generally 

 due to some illness or accident which affected one of 

 the twins. The gradual influence of a number of small 

 causes seemed to have very little effect in this respect. 

 * [Cases are, however, known in which twins, apparently 

 ' identical ' in most characters, differ in one or more which are 

 certainly congenital.] 



