286 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. 
‘In not a single instance,’ Galton writes, ‘ have I met 
