sand each, and it becomes infinitesimal. You have practically each time not only a synerasis but an idio-syncrasis as well.
Now, a human being is the product of innumerable elements, derived directly from two parents, and indirectly from an infinity of earlier ancestors; elements not of two orders only, but of infinite orders; combined together, apparently, not on the principle of both contributing equally to each part, but of a sort of struggle between the two for the mastery in each part. Here, elements derived from the father's side seem to carry the day; there, again, elements derived from the mother's side gain the victory; and yonder, once more, a compromise has been arrived at between the two, so that the offspring in that particular part is a mean of his paternal and maternal antecedents. Under such circumstances, absolute equality of result in any two cases is almost inconceivable. It would imply absolute equality of conditions between myriads of jarring and adverse elements, such as we never actually find in nature, and such as we can hardly believe possible under any actual concrete circumstances.
The case of twins comes nearer to such exact equality of conditions than any other with which we are acquainted. Here, the varying health and vigor of the two parents, or the difference between their respective functional activities at two given times, are reduced to a minimum; and we get in many instances a very close similarity indeed. Yet even among twins, the offspring of the same father and mother, produced at the same moment of time, there are always at least some differences, mental and physical; while the differences are occasionally very great. A competent observer, who knew the Siamese twins, informed me that differences of disposition were quite marked in their case, where training and after-circumstances could have had little or nothing to do with them, inasmuch as both must have been subjected to all but absolutely identical conditions of life throughout. One was described as taciturn and morose, the other as lively and good-humored. Whether anything of the same sort has been noticed in the pair of negro girls called the Two-headed Nightingale, I do not know, but, to judge from their photographs, there would seem to be some distinct physical diversities in height and feature. We can only account for these diversities in twins generally by supposing that in that intimate intermixture of elements derived from one or other parent, which we have learned from Darwin, Spencer, and Galton, takes place in every impregnation of an ovum, slightly different results have occurred in one case and in the other. To use Darwin's phraseology, some gemmules of the paternal side have here ousted some gemmules of the maternal side, or vice versa; to use Mr. Spencer's (which to my judgment seems preferable), the polarities of one physiological unit have here carried the day over those of another.
But why under such nearly identical conditions should there be such diversity of result? Let us answer the question by another: