that they differ at all. The close agreement with stature, forearm, span and other physical characteristics is clear.
A criticism which may occur to the reader is that these resemblances are purely spurious, and due to the fact that husband and wife are not likely to be buried in the same ground unless they die within a short period of each other. This is precisely what would occur in districts with a shifting population. This very difficulty was anticipated. All urban churchyards were excluded because of the heterogeneous and transitory nature of the population. "In most rural districts, on the other hand, with a stable population, there is a very strong feeling—amounting in the case of the Yorkshire Dales almost to a superstition—that husband and wife must share the same grave." In view of the careful selection of localities and the agreement of results secured from Quaker archives with those from the gravestones, it seems clear that the resemblance can not be dismissed as purely spurious.
Duration of life is doubtless dependent upon environment as well as upon constitution. Slight differences in the healthfulness of the several parishes of a district might promote or oppose fullness of years in men and women alike. If this were true, the lumping together of the records from a number of churchyards would result in a correlation for duration of life, whether there be any real assortative mating or not. If this criticism be valid, random pairs of husbands and wives from the same parishes lumped together to form a table for the whole district, should show correlations as high as those for actual married pairs. The correlation really found was sensibly zero, demonstrating that local environmental conditions can not explain the results.
But within the same general environment, members of a family are exposed to a set of conditions peculiar to themselves. In a city block or country parish food, temperance, sanitation, risk of zymotic disease, and physical and mental habits differ greatly from family to family. In addition to the physical and social environment common to man and wife, there is also the fact that the death of one member of the pair has a profound effect on the other. Financial and associated domestic changes are often due directly to this cause, to say nothing of the overstrain of care during long illness or the shock of sudden death. May not the similarity in the duration of life of husband and wife be a consequence of domestic environment? This point was investigated by methods rather too complicated for explanation here, but which indicate that the sameness of home conditions can not account for the relationship.
Until further evidence is available, we must, therefore, conclude that there is a real, though assuredly unconscious and quite indirect, assortative mating for duration of life. Nor is this to be marvelled at, considering the results already noted for normal and especially for abnormal bodily characters.