244 THE ORIGIN AND EVOLUTION OF LIFE 



life, and finally from the leaping over the ground of the kan- 

 garoo into the incipiently specialized arboreal phase of the 

 tree kangaroo. In the evolution of the tree kangaroos adap- 

 tation is certainly not limited by the inherent tendencies of 

 the heredity-chromatin to evolve in certain directions. The 

 physicochemical theory of these remarkable alternate adap- 

 tations is that an animal leaving the terrestrial habitat and 

 taking on arboreal habits initiates an entirely new series of 

 actions, reactions, and interactions with its physical environ- 

 ment, with its life environment, in its body cell and individual 

 development, and, in some manner entirely unknown to us, in 

 its heredity-chromatin, which begins to show new or modified 

 determiners of bodily character. That natural selection is 

 continuously operating at every stage of the transformation 

 there can be no doubt. 



One interpretation which has been offered up to the pres- 

 ent time of the mode of transformation of a terrestrial into an 

 arboreal mammal is through a form of Darwinism known as 

 the "organic selection" or "coincident selection" hypothesis, 

 which was independently proposed by Osborn,' Baldwin, and 

 Lloyd Morgan, namely: that the individual bodily modifications 

 and adaptations caused by growth and habit (while not them- 

 selves heritable) would tend to preserve the organism during the 

 long transition into arboreal life; they would tend to nurse the 

 family over the critical period and allow time to favor all pre- 

 dispositions and tendencies in the heredity-chromatin toward 

 arboreal function and structure, and would tend also to elim- 

 inate all structural and functional predispositions in the hered- 

 ity-chromatin which would naturally adapt a mammal to life 

 in any one of the other habitat zones. This interpretation is 

 consistent with our law that selection is constantly operating 



•Osborn, H. F., 1S97. 



