[271]. l. c., p. 92.
[272]. [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, p. 325.
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. A very 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 unaltered 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 physical 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 Kleinwächter’s observations 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 differences, 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 nurture is strongly confirmed. Mr. Galton writes, ‘I have not a single case in which my correspondents speak of originally dissimilar characters having become assimilated through identity of nurture. The impression that all this evidence leaves on the mind is one of wonder whether nurture can do anything at all, beyond giving instruction and professional training.’
The fact that twins produced from a single ovum seem to be invariably of the same sex is in itself extremely interesting, for it proves that the sex of the individual is predetermined in the fertilized ovum.—E. B. P.]
[273]. Fol, Recherches sur la fécondation et le commencement de l’hénogénie: Genève, Bâle, Lyon. 1879.
[274]. Born, ‘Ueber Doppelbildungen beim Frosch und deren Entstehung.’ Breslauer ärztl. Zeitschrift, 1882.
VII.