Woods: Significant Evidence for Mental Heredity 



109 



other "atmospheric" conditions are also 

 different. We cannot experiment ad 

 libitum, as the agriculturist does, but 

 there are already certain curious collec- 

 tions of data either discovered or com- 

 piled which give us a chance to test 

 the same heredity under shifting cir- 

 cumstances, or the effect of a varying 

 germ-plasm developing under uniform 

 conditions. One of the first instances 

 to be utilized and one of the most 

 interesting is the evidence from various 

 sorts of twins. It is known that there 

 are two sorts of twins. The true or 

 "identical" twins are developed from a 

 single original cell which at some very 

 early stage divided to form two indivi- 

 dual beings. These "identical" or 

 "duplicate" twins have a nearly (though 

 never an absolutely) identical germ- 

 plasm or heredity. Ordinary brothers 

 and sisters are not nearly as identical 

 in their origin. They spring from cells 

 necessarily somewhat similar though 

 not nearly so much so. Fortunately 

 for our knowledge of heredity it often 

 happens that brothers and sisters are 

 born at the same time, who are also 

 called twins, but are in a different 

 category from the true or identical 

 twins. These are on the same basis for 

 hereditary resemblance as ordinary 

 brothers and sisters, and are known as 

 "fraternal" twins. As far as we can 

 see, home environment for the "iden- 

 tical" twins is no more identical than 

 for the "fraternal" twins. In both cases 

 there are slight differences which tend 

 to become greater from the start, but 

 not more in one than the other. If 

 they are of the same sex the parents 

 commonly do not know which variety 

 they are. As the twins grow up they 

 may remain very much alike in mental 

 characteristics or they may diverge 

 as much as ordinary brothers and 

 sisters. 



THE TWO KINDS OF TWINS 



Thorndike- presents some evidence 

 to discredit the commonly accepted 

 view that there are absolutely two kinds 

 of human twins or, rather, that any con- 

 siderable percentage of twins develop 



from one fertilized egg. He finds all 

 degrees of gradation in his measure- 

 ments of New York school children, but 

 I am inclined to think that there are 

 two kinds of twins from the embryol- 

 ogist's point of view, in spite of the 

 gradation in measurements. We know 

 that there are in the widest extreme two 

 kinds. Those of different sex must 

 come from two distinct ova, each 

 fertilized by a separate sperm. Our 

 present knowledge of sex causation, 

 and of the accessory chromosome, 

 makes it almost certain that those of 

 opposite sex are from two ova and two 

 spermatazoa. If this is true for oppo- 

 site sexes, why not for the same sex? 

 There should be. as many more of the 

 same sort who happen to be of the 

 same sex, boy and boy, or girl and girl. 

 We also know from embryological 

 evidence that it is possible to have two 

 persons derived from the single fer- 

 tilized ovum. No one denies this. 

 Therefore, what seems to me most 

 likely to explain all the facts is that in 

 the case of identical twins, if the single 

 cell splits early and uniformly into two 

 almost identical parts, we get the closest 

 approximation to identity. If the 

 division takes place in the four or eight 

 cell stage or later, or if for any reason 

 the chromosomes are not so evenly 

 divided, even if the separation took 

 place in the one cell stage, then the 

 adult resemblance would be less strong. 

 The fact that there is lack of agree- 

 ment on this i3oint does not make 

 the evidence from twins without signifi- 

 cance for mental heredity, but it does 

 call into question whether we can be 

 sure that we are dealing with a' very 

 much more identical germ-plasm in 

 one class of cases than in another. 

 Therefore we cannot very well apply 

 all of the tests of "heredity the same," 

 "environment changed" that I advo- 

 cated in what I call the method of 

 differences. 



Some comparison of this sort can, 

 however, be made. For instance, twins 

 between 9 and 14 years of age were 

 tested in two ways: first, in their 

 abihty to add and multiply figures. 



2 Measurements of Twins, Archives of Philosophy, Psvchologv and Scientific Methods, No. 1, 

 New York, 1905. 



