ORTHOGENESIS 337 



Both Mr. Cunningham and Prof. Eimer are ever emphatic 

 in pointing out that physiological change precedes morpho- 

 logical. Thus the former tells us: 



There is a climbing kangaroo in Papua which shows so little adapta- 

 tion of structure to the climbing habit, that no naturalist would believe 

 from the mere study of its body that it lived in trees. But as a matter 

 of fact it does live entirely in trees. 



What Mr. Cunningham here wishes to convey, of course, 

 is that this kangaroo has not yet had time to adapt 

 its structure to the climbing habit, but will presently 

 do so, and that we have here, therefore, an indication of how 

 habit precedes structure, and how orthogenesis is explainable 

 on physiological grounds. In reality, however, orthogenesis is 

 here to be explained by symbiogenesis. On my symbiogenetic 

 view of evolution, moreover, there is nothing to necessitate the 

 belief that every habit formed (even so useful a one as climb- 

 ing) must needs cause " adaptation " of the whole structure to 

 a conspicuous degree. The better the habit the less need it 

 lead to an extreme determination of the organism in accordance 

 with the particular habit, and vice versa in the case of bad 

 habits. 



Now, tree-climbing in this instance is essentially con- 

 nected with arboreal life and with cross-feeding. The 

 kangaroo, therefore, may be said to be on the path of symbio- 

 genesis, and capable of relying on symbiogenetic momenta 

 generally, and not needing to specialise in extreme deter- 

 mination, i.e., in dependence on a particular prey, as, for 

 instance, did the sabre-toothed tiger, whose teeth-formation 

 rendered it impossible for it to prey on any other animal 

 when it had destroyed the whole species of giant armadillo. 



The kangaroo, because of its physiologically sound, i.e., 

 symbiogenetic feeding-habits, could afford to be arboreal with- 

 out a highly specialised adaptation for tree climbing, which 

 might have rendered subsequent terrestrial adaptation difficult 

 or impossible. It presents the example of an animal wisely 

 preservative of symbiotics, showing a remarkable " survival of 



