DIMINISHING ENVIRONMENTAL INFLUENCE 329 



On the other hand, it is said that arctic explorers experience slug- 

 gishness 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. jSTormally, bone like the epidermis is being constantly re- 

 made 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 modifica- 

 tions 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 39 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. 40 Our general 

 knowledge concerning human twins supports this view. A r ery fre- 



39 " Senescence and Rejuvenation," Journal of Physiology. Vol. XII., No. 2, 

 1891. 



40 Jour, of Experimental Zooh. 1909. • 



VOL. LXXVI. — 23. 



