26 SUMMARY OF CURKENT RESEARCHES RELATING TO 



to the friction ridges, the muscles and the arteries ; some similarity in 

 the distribution of nerves ; and practically no resemblance in regard tfi 

 the veins. Of the forty-nine points in which one or another of the 

 three hands differed from the other two, the twins were alike in thirty- 

 five, while each twin differed from the other twin and agreed with the 

 control in seven different points. If 50 p.c. be deducted for indirect 

 effects of .the extra digits and for racial factors, the twins would still 

 resemble each other two and a half times as much as either one resembled 

 the control. It would seem therefore that heredity is the principal 

 factor involved in the variations in question. J. A. T. 



Resemblance and Difference in Twins. — C. H. Daxforth {Journ. 

 of Heredity, lt»ll), 10, No. 'J, 1-6). The close similarities of some twins 

 and the differences of others are commonly explained on the assumption 

 that there are two kinds of twips, those arising froui a single ovnm, 

 and those derived from two separate and independent ova, the former 

 being closely alike, the latter often very unlike. But Thorndike has 

 found that many twins that are undoubtedly biovular in origin resemble 

 each other more than they resemble their other brothers and sisters. 

 Three questions are asked : — 1. Are the resemblances of uniovular 

 twins confined to certain traits, or does the similarity pervade their 

 whole being ? 2. Apart from environmental diffei'ences, how great is 

 the similarity that may be expected l)etween biovular twins ? 3. How 

 great are the differences that may be expected to appear in uniovular 

 twins ? To the first question it is answered that the resemblances are 

 fundamental ; but this may be 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 indiviilual his 

 character. This will account for cases where in spite of close similarity 

 there are differences in some traits. To the second question it is 

 •answered that biovular twins will on the whole resemble each other to a 

 degree about equal to the average for all children of the same family, 

 but this may be increased by the sameness of the nurture. The data, 

 however, are apt to lead astray, because twins who do not closely 

 resemble one another tend to be overlooked. Differences in uniovular 

 twins may l)e due to the same cause as the differences between the two 

 sides of the body. Wilder has suggested that if a twin is developed 

 from part of an ovum while the ordinary individual is developed from 

 all of it, twins should be more symmetrical than ordinary individuals. 



J. A. T. 



Heredity in Rabbits, Rats and Mice.— W. E. Castle (Publications 

 Carnegie Inst. Washington, 11)19, 288, 1-56, 3 pis.) 1. The hooded 

 character in piebald rats is a Meudelizing character, but selection, either in 

 the plus direction or in the minus direction, is marked l)y a consistent 

 advance in the mean of the character. It appears, therefore, that selec- 

 tion can permanently change racial characters which Mendelize. Crossing 

 the plus-selected hooded race with the non-hooded wild race reduces the 

 grade of the hooded character, undoing in a measure the work of selec- 

 tion. Crossing the minus-selected race with the same wild race resulta 



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