On the other hand, it is said that arctic explorers experience sluggishness of the mind during the long winter night, as a direct result of the darkness. I do not know how far this is true, but in order to show that this is contradictory to my generalization it would be necessary to prove that the effect is greater upon the minds of men than upon the minds of domestic animals, and greater upon the minds of the leaders of the parties than upon the crews. It would seem improbable that such is the case. Moreover, I am informed by Captain Bartlett of the Roosevelt, that if the men are busy with duties, and if their minds are occupied to the usual extent, no such depressions occur.
With regard to the influences of direct contact on different tissues, I have already noted that pressure produces an easy modification upon the outer skin. Prolonged pressure will also produce noticeable changes in the shapes of growing mammalian bones, but it is probable that even greater modifications might be produced on the skeletons of lower animals. Normally, bone like the epidermis is being constantly remade by proliferation of young cells from the growing layers. In this respect it differs from nerve tissue, the cells of which cease division in early embryonic life.
Boas has recently announced that he has found evidence that the head forms of the children of Hebrew and Sicilian immigrants who come to the United States tend to approach the American type, as a direct result of some mysterious influence of the environment. This he assumes to be of suggestive value to the psychologist and sociologist. He fails to take into account the great anatomical and embryological differences between bone tissue and cerebral nerve tissue. The real deduction from all this work (if indeed it should be confirmed) is that it is easier to modify a bone than it is a brain.
If we consider the effects of different kinds of feeding upon higher animals, as contrasted with the lower, it is evident that the modifications brought about in this way are much less striking among the higher. The linear dimensions of lower organisms may be altered from two to twenty fold. On the other hand, there appears to be an inherent tendency for mammals to grow to a certain definite size within narrow limits. Minot[1] has shown that the rate of growth of guinea-pigs may be artificially altered, but that there is nevertheless a strong tendency for guinea-pigs to grow to a certain size, and that they make up in later stages what they lose in the younger; or if there is an extra increment in the younger stages this is compensated for, later on. This is confirmed by F. B. Sumner for the white mouse.[2] Our general knowledge concerning human twins supports this view. Very fre-
