404 



The Journal of Heredity 



pairs show no such unusual similarity. 

 The resemblance between similar pairs 

 is never quite complete and practically 

 every pair is found to differ more or 

 less widely in at least one respect. On 

 the whole one might gain the impres- 

 sion that while absolute identity is not 

 attained, rather marked similarity is, 

 with rare exception, the rule. 



DIFFERENCES AND SIMILARITIES EX- 

 PLAINED 



It may now be of interest to seek an 

 explanation for these likenesses and 

 differences, and particularly to try to 

 explain the intermediate grades of re- 

 semblance 



The similarities of some twins and 

 the differences of others are commonly 

 explained on the familiar assumption 

 that there are two classes of twins, 

 those derived from a single ovum and 

 those derived from two separate and 

 independent ova.^ The former, having 

 identical heredity, are similar, while the 

 latter, arising from different ova and 

 different sperms, are dissimilar. Both 

 of these types furnish strong evidence 

 for the overwhelming potency of fac- 

 tors of heredity as compared with those 

 of environment. In the one case the re- 

 semblance persists, as can sometimes be 

 shown, in spite of dissimilar environ- 

 ments, while, in the other, identity of 

 home surroundings, food, and even 

 clothes fails to lessen inborn differences. 



It cannot be maintained, however, 

 that heredity is the sole factor in de- 

 termining the characteristics of the 

 individual. Thorndike^ has found, for 

 example, that many twins which are 

 undoubtedly biovular in origin resemble 

 each other more than they resemble 

 their other brothers and sisters. While 

 this is not Thorndike's conclusion, the 

 observation might be interpreted as 

 indicating the possibility of an environ- 

 mental influence. Since it is also to be 



observed <^hat uniovular twins are never 

 quite "identical," it is conceivable that 

 their differences might be due to en- 

 vironmental factors. How far such 

 similarities in the one case and differ- 

 ences in the other are in reality directly 

 induced by the environment cannot be 

 easily answered. A few considerations 

 bearing on this question may be of some 

 interest. In this connection there are 

 three questions that it may be profitable 

 to consider: (1) Are the resemblances 

 of uniovular twins confined to certain 

 traits or does the similarity pervade 

 their whole being? (2) Apart from 

 environmental influences, how great is 

 the similarity that may be expected be- 

 tween biovular twins? (3) How great 

 are the differences that may be ex- 

 pected to appear in uniovular twins ? 



In regard to the first of these ques- 

 tions it will at once be apparent that 

 many of the so-called superficial re- 

 semblances are in reality expressions of 

 more deep-seated similarities. The 

 color of the hair and eyes is due to 

 fundamental factors in the make-up of 

 the individual. The gait and many other 

 peculiarities that might seem superficial 

 are the product of the condition of the 

 muscular and nervous systems. Some 

 few dissections of twins have been 

 made, and these revealed remarkable 

 similarity in the configuration of the 

 brain^ and in the muscles and blood 

 vessels.* So there is every reason to 

 believe that the resemblances between 

 twins are fundamental, involving all 

 parts and systems. But while every part 

 may be concerned, there is some indi- 

 cation that the total resemblance is due, 

 not to some general quality that the two 

 individuals have in common, but rather 

 to the sum of the many similar special 

 traits which, added together, give the 

 individual his character. On this as- 

 sumption we can understand at least 

 a part of the cases in which there is 



1 For a fuller discussion of this matter the reader may referred to an article in this 

 journal, vol. vii, No. 5, pp. 195-202, "Is Twinning Hereditary?" 



2 Archives of Philosophy, Psychology and Scientific Methods, No. 1, 1905. 



3 Sano F. : "The convolutional pattern of the brains of identical twins: a study on 

 hereditary resemblance in the furrows of the cerebral hemispheres." Phil. Trans. Roy. See, 

 Lon., Series B, vol. ccviii. pp. 3761, \91C. 



^Danforth, C. H. : "A comparison of the hands of a pair of polydactyl negro twins." 

 Amer. Jour. Physical Anthropology, vol. ii, pp. 147-165, 1919. 



