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eclipses and planetary movements in spite of this fundamental 

 misconception. Mysteries and discrepancies remained, how- 

 ever, until students of the heavenly bodies were willing to 

 admit that the sun was the center of the system and that the 

 earth revolved like her sister planets. 



If adaptations were the only evolutionary phenomena in need 

 of explanation, the doctrine of environmental causes might serve 

 scientific purposes for as many centuries as the Ptolemaic 

 astronomy, but it has become very apparent that many organic 

 changes are going on which have no connection with adapta- 

 tion, and which would not be explained by selection, even if 

 everything claimed for it were to be admitted. 



To think of species as normally in motion will be found very 

 difficult, no doubt, by those who have been so long accustomed 

 to take it for granted that they are normally at rest. The dif- 

 ficulties of readjustment are still further increased by the fact 

 that the available technical language and customary forms of 

 expression have been elaborated for the exposition of the static 

 doctrine of environmental causation, and lend themselves only 

 with difficulty to the presentation of the opposite doctrine, that 

 species are normally in motion. 1 Many distinctions formerly 

 considered of value now appear to have little significance. 

 Many things are readily explainable which seemed utterly 

 mysterious before, and many new problems can be approached 

 which have hitherto appeared quite inaccessible. 



Since the time of Darwin a long and varied series of amend- 

 ments and supplements have been proposed for the doctrine 

 of natural selection, and no end of diversity of individual 

 opinion has existed among biologists regarding the adequacy 

 and relative significance of the various factors and forms of 

 selection. The kinetic theory enables us to look beyond all 

 this cloud of discussion and to perceive that selection is not 

 merely inadequate as the cause of evolution ; it is not an evo- 

 lutionary cause at all, in the concrete physiological sense ; it 

 does not set evolution in'motion, nor keep it going. 



1 Three classes of difficulties attend the progress of science, the concrete diffi- 

 culties of ascertaining facts, the conceptual difficulties of interpreting them, and 

 the philological difficulties of describing the new facts and the concepts in terms 

 of general intelligibility. The problems of expression are often quite as serious 

 as the others, and quite as worthy of scientific study. 



