This remarkable case appears to militate against the conclusion, drawn a few pages back, that the increase of a peculiarity by coincidence of "spontaneous variations" in successive generations, is very improbable; and that the special superiorities of musical composers cannot have thus arisen. The reply is that the extreme frequency of the occurrence among so narrow a class as that of musical composers, forbids the interpretation thus suggested.
I omitted to name here a cause which may be still more potent in producing irregularity in the results of cousin-marriages. So far as I can learn, no attempt has been made to distinguish between such results as arise when the related parents from whom the cousins descend are of the same sex and those which arise when they are of different sexes. In the one case two sisters have children who intermarry; and in the other case a brother and a sister have children who intermarry. The marriages of cousins in these two cases may be quite dissimilar in their results. If there is a tendency to limitation of heredity by sex—if daughters usually inherit more from the mother than sons do, while sons inherit more from the father than from the mother, then two sisters will on the average of cases be more alike in constitution than a sister and a brother. Consequently the descendants of two sisters will differ less in their constitutions than the descendants of a brother and a sister; and marriage in the first case will be more likely to prove injurious from absence of dissimilarity in the physiological units than marriage in the second. My own small circle of friends furnishes evidence tending to verify this conclusion. In one instance two cousins who intermarried are children of two sisters, and they have no offspring. In another the cousins who intermarried are children of two brothers, and they have no offspring. In the third case the cousins were descendants of two brothers and only one child resulted.
A propos of this sentence one of my critics writes:—"I cannot find in this book the statement as first made that the 'life of an individual is maintained by the unequal and ever-varying actions of incident forces on its different parts.' Recent physiological work offers a startling example of the statement."
To the question contained in the first sentence the answer is that I have not made the statement in the above words, but that it is implied in the chapter entitled "The Degree of Life varies as the Degree of Correspondence," and more especially in [§ 36], which, towards its close, definitely involves the statement. The verifying evidence my critic gives me is this:—
"Prof. Sherrington has shown that if the sensory roots of the spinal nerves are cut one by one there is at first no general effect produced. That is to say, the remainder of the nervous system continues to function as before. This condition (lack of general effect) persists until about six pairs have been cut. With the severance of the seventh pair, however, the whole central nervous system ceases to function, so that stimulation of intact sensory nerves produces no reflex action. After a variable period, but one of many hours duration, the power of functioning is recovered. That is to say, if the sensory impulses (from the skin, &c.) reaching the central nervous system are rapidly reduced in amount, there comes a point where those remaining do not suffice to keep the structure 'awake.' After a time, however, it adjusts itself to work with the diminished supply. Similarly Strumpell describes the case of a boy 'whose sensory inlets were all paralyzed except one eye and one ear.' When these were closed he instantly fell asleep."
Fifty years before the discovery of the Röntgen rays and those habitually emanating from uranium, it had been observed by Moser that under certain conditions the surfaces of metals receive permanent impressions from appropriate objects placed upon them. Such facts show that the molecules of substances propagate in all directions special ethereal undulations determined by their special constitutions.