144 PROBLEMS OF EVOLUTION 



herent tendency towards evolution on particular lines ? " The 

 answer must be a decisive negative. Had it been so, we might 

 have expected identical forms to be developed in different parts 

 of the world, and this is very far from being the case. Aus- 

 tralian mammals have not progressed beyond the marsupial stage. 

 At every step in evolution there must be the coincidence of a suitable en- 

 vironment, to select the variation in question for survival, or else 

 it will be lost. And though the nature of the organism limits 

 the angle of deviation, it sets no limit to the eventual amount of 

 divergence. The evolution of species is not like the driving of 

 pigs along a road railed in on either side and with no roads lead- 

 ing off. There are endless branching roads, but they slope 

 away at a small angle, and to those individuals that choose unad- 

 visedly Natural Selection is perpetually giving the coup de grace, 

 so that by no means all variations prove to be adaptations. But 

 all this does not prevent us from holding that, given protoplasm 

 and given the physical conditions that have prevailed upon 

 the earth since the appearance of life, organisms, having in 

 various combinations the main characteristics of those that exist, 

 were bound to appear. Thus much may be allowed. The 

 particular forms evolved are due to the interplay of organism 

 and environment, the latter consisting largely of competing 

 organisms. Much, then, was inevitably left to chance. On the 

 one side there has been a succession of grim necessities, on the 

 other a succession of chance adaptations. 



Easiness of We must now consider adaptations further to see how chance can 

 adaptation | iave brought them about. If we begin at the wrong end, as people 

 are too apt to do, it seems an insoluble problem. Even such a 

 familiar bird as the domestic duck is a mass of adaptations, and 

 that it should have fitted into its elaborate environment to a nicety 

 seems too great a miracle for a system in which chance plays a 

 part. It will be much better to begin at the very base and picture 

 to ourselves an unspecialised animal in an unspecialised environ- 

 ment. Our primitive animal, a blob of protoplasm, must be able 

 (l) to get oxygen from the water in which he lives ; (2) to get 

 protoplasm for food ; (3) must propagate his kind ; (4) must 

 stand changes of temperature j (5) he may have to kill or escape 



