392 O.V THE NUMBER OF POLAR BODIES AND [VI. 



well known tiiat occasionally some of the children of the same 

 parents appear to be almost exactly alike, but such children 

 are without exception twins, and there is every reason to 

 believe that thc}'^ have been derived from the same Q:%g. In 

 other words, the two children are exactly alike because they 

 have arisen from the same egg-cell, which could of course only 

 contain a single combination of ancestral germ-plasms, and 

 therefore of hereditary tendencies \ The factors which by 



^ [The similar conclusion that identical ova lead to the appearance of 

 identical individuals was drawn from the same data by Francis Galton in 

 1875. See ' The history of the Twins, as a criterion of the relative 

 powers of Nature and Nurture,' by Francis Galton, F. R.S., Journal of 

 the Anthropological Institute, 1875, p. 391 ; also by the same author, 



* Short Notes on Heredity, etc. in Twins,' in the same Journal, 1875, 



The author investigated about eighty cases of close similarity between 

 twins, and was able to obtain instructive details in thirty-five of these. 

 Of the latter there were no less than seven cases ' in which both twins 

 suffered from some special ailment or had some exceptional peculiarity ; ' 

 in nine cases it appeared that ' both twins are apt to sicken at the same 

 time ; ' in eleven cases there was evidence for a remarkable association of 

 ideas ; in sixteen cases the tastes and dispositions were described as 

 closely similar. These points of identity are given in addition to the 

 more superficial indications presented by the failure of strangers or even 

 parents to distinguish between the twins. Avery interesting part of the 

 investigation was concerned with the after-lives of the thirty-five twins. 



* In some cases the resemblance of body and mind had continued un- 

 altered up to old age, notwithstanding very different conditions of life,' 

 in the other cases ' the parents ascribed such dissimilarity as there was, 

 wholly, or almost wholly, to some form of illness.' 



The conclusions of the author are as follows : ' Twins who closely 

 resembled each other in childhood and early youth, and were reared 

 under not very dissimilar conditions, either grow unlike through the 

 development of natural characteristics which had lain dormant at first, 

 or else they continue their lives, keeping time like two watches, hardly 

 to be thrown out of accord except by some ph\'sical jar. Nature is 

 far stronger than nurture within the limited range that I have been 

 careful to assign to the latter.' And again, ' where the maladies of twins 

 are continually alike, the clocks of their two lives move regularly on, and 

 at the same rate, governed by their internal mechanism. Necessitarians 

 may derive new arguments from the life histories of twins.' 



The above facts and conclusions held for twins of the same sex, of 

 which at any rate the majority are shown by Kleinwiichtcr's observa- 

 tions to have been enclosed in the same embryonic membranes, and 

 therefore presumably to have been derived from a single ovum ; but in 

 rarer cases the twins, although also invariably of the same sex, were 

 marked by remarkable diflcrences. greater than those which usually 

 distinguish children of the same family. Mr. Galton met with twenty of 

 these cases. In such twins the conditions of training, etc. had been 

 as similar as possible, so that the evidence of the power of nature over 



