70 EVOLUTION AND ANIMAL LIFE 



the tiny plume behind the ear. Any character of the parent 

 stock, which may prove harmful under new relations, will be 

 eliminated by natural selection. Those especially helpful will 

 be intensified and modified. But the great body of characters, 

 the marks by which we know the species, will be neither helpful 

 nor hurtful. These will be meaningless streaks and spots, 

 variations in size of parts, peculiar relations of scales or hair or 

 feathers, little matters which can neither help nor hurt, but 

 which have all the persistence heredity can give. 



In regard to natural selection our knowledge seems positive. 

 In regard to most other factors of organic evolution we have 

 to deal so far not with clearly demonstrated facts but with 

 "probabilities of a higher or lower order/' their value to be 

 ultimately shown by experiment. 



In this connection the following words of Dr. Edwin Grant 

 Conklin are very pertinent : 



"On the whole, then, I believe the facts which are at present at our 

 disposal justify a return to the position of Darwin. Neither Weis- 

 mannism nor Lamarckism alone can explain the causes of evolution. 

 But Darwinism can explain those causes. Darwin endeavored to show 

 that variations, perhaps even adaptations, were the result of extrinsic 

 factors acting upon the organism, and that these variations or adap- 

 tations were increased and improved by natural selection. This is, I 

 believe, the only ground which is at present tenable, and it is but 

 another testimony to the greatness of that man of men that, after 

 exploring for a score of years all the ins and outs of pure selection and 

 pure adaptation, men are now coming back to the position outlined 

 and unswervingly maintained by him." 



Finally we ought not to suppose that we have already 

 reached a satisfactory solution of the evolution problem, or 

 are, indeed, near such a solution. 



"We must not conceal from ourselves the fact," says Roux, "that 

 the causal investigation of organisms is one of the most difficult, if 

 not the most difficult, problems which the human intellect has at- 

 tempted to solve, and that this investigation, like every causal 

 science, can never reach completeness, since every new cause ascer- 

 tained only gives rise to fresh questions concerning the cause of this 

 cause." 



