3 88 THE POPULAR SCIENCE MONTHLY. 



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 or- 

 ders ; combined together, apparently, not on the principle of both con- 

 tributing 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 antece- 

 dents. 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 condi- 

 tions than any other with which we are acquainted. Here, the vary- 

 ing 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 in- 

 deed. 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 : 



