218 ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 34 



the eye without a shudder. He sought refuge, as did Cope and many 

 others, in the inherited effects of use and disuse as an aid to natural 

 selection, but this refuge is now denied us, for the evidences from 

 genetics are conclusive that such effects are not inherited. 



A solution that has found favor with many geneticists lies in 

 the vastly greater duration of past time than was formerly allowed 

 for organic evolution. Darwin estimated that past evolution must 

 have required something like 400 million years. Lord Kelvin speak- 

 ing for the physicists of his day would allow him not more than 100 

 million years. But the physicists, astronomers, and geologists now 

 say that the earth was ready for life at least 1,000 million years 

 ago, and geneticists console themselves with the thought that given 

 almost infinite time and almost an infinitude of mutations, almost 

 anything could happen. But after all they cannot help feeling that 

 this is not a satisfactory solution of the vast problem of fitness — 

 at present by far the greatest problem of biology. 



Another possible solution of this problem was first pointed out 

 by Roux in his hypothesis of " the struggle of the parts " and by 

 Weismann in his doctrine of " intrapersonal selection." In short, 

 selection acts not only on developed organisms and the correlations of 

 all their parts and organs, but much more on embryos, germ cells, and 

 combinations of chromosomes, genes, and mutations; indeed, it acts 

 to preserve a proper balance between all structures and processes of 

 the organism. Thus unfavorable combinations may be eliminated 

 at their beginnings, and the more unfavorable they are the earlier they 

 are eliminated. 



I have proposed (1921) a still further application of the selec- 

 tion principle to all the reactions of living things. We know that 

 all organisms are differentially sensitive ; that is, they move or grow 

 toward certain sources of stimuli and away from others, and in 

 general they respond positively to stimuli which we would call 

 pleasant or satisfactory, and negatively to those which we call un- 

 pleasant or unsatisfactory. In short, they are generally able to 

 differentiate and select between that which is satisfactory and that 

 which is not. No one can at present explain this property of life, 

 but apparently it is a general characteristic of all living things. It 

 characterizes the behavior of germ cells and embryos as well as adult 

 organisms. It is the basis of that form of behavior known as " trial 

 and error " ; it is fundamental to all learning and is the beginning 

 of intelligence and wisdom in man as well as in higher animals. 

 This capacity to differentiate and select is not unlike the " archaes- 

 thetism " of Cope, and it is at bottom an extension of the selection 

 ]]irinciple to the reactions of organisms — but with this difference, 

 that whereas in Darwinian selection the selector or eliminator was 



