408 



The Journal of Heredity 



same respects and to the same degree 

 as the two sides of the body differ in 

 ordinary individuals. 



Whether the fact that a twin is de- 

 veloped from only a part of the ovum 

 while the ordinary individual is devel- 

 oped from all of it makes the two sides 

 of the twin any less variable is not defi- 

 nitely known, but Professor Wilder^ 

 makes the interesting suggestion that it 

 does and that twins are therefore apt 

 to be more symmetrical than ordinary 

 individuals. Moreover, it is possible, so 

 far as we can see, that occasionally — 

 although rarely, no doubt — they may 

 differ not in one or two particulars but 

 in many, giving uniovular twins which 

 are decidedly different. These facts 



help us to understand, if not to explain,, 

 why uniovular twins are not really 

 identical in the strict sense of the word. 

 The theoretical considerations briefly 

 set forth in the preceding paragraphs 

 may serve to account for most of the 

 resemblances and differences actually 

 observed among twins. They help us 

 to understand why uniovular twins are 

 not absolutely identical and why biovu- 

 lar twins are often very similar. In the 

 same facts we find an explanation for 

 those intermediate grades of resem- 

 blance that often prove so puzzling but 

 which, after all, seem to be due more 

 to the inherent constitution of the germ 

 plasm than to influence of the environ- 

 ment. 



TWINS WHO INHERITED NIGHT BLINDNESS 



Mr. E. C. McCurdy and Mr. R. G. McCurdy feel very strongly that all identical twins are 

 fonder of each other than ordinary brothers and sisters. The McCurdy twins are also fond of 

 the same things, and they like children and animals, enjoy the same kind of diversions, foods 

 and intellectual interests, and they share an abhorance of bugs, spiders and snakes. They 

 both suffer from night blindness and neither can get about at night, although they both have 

 a good sense of direction. Their handwriting is very different. (Fig. 13.) 



8 Wilder H. H. : "Palm and Sole Studies." Biological Bulletin, vol. xxx, 1916. For a. 

 further discussion of s>ir.metry in twins see "The Biology of Twins," by H. H. Newman,. 

 University of Chicago Press, 1916. 



