44 BIOLOGY AND ITS MAKERS 



may be admitted, there is still abundant evidence of evolu- 







tion without mutation. 



Reconciliation of Different Theories. All this is leading 

 to a clearer appreciation of the points involved in the dis- 

 cussion of the theories of evolution; the tendency is not for 

 the breach between the different theories to be widened, but 

 for evolutionists to realize more fully the great complexity 

 of the process they are trying to explain, and to see that no 

 single factor can carry the burden of an explanation. Muta- 

 tion is not a substitute for natural selection, but a cooperating 

 factor; and neither mutation nor natural selection is a sub- 

 stitute for the doctrine of the continuity of the germ-plasm. 

 Thus we may look forward to a reconciliation between 

 apparently conflicting views, when naturalists by sifting 

 shall have determined the truth embodied in the various 

 theories. One conviction that is looming into prominence is 

 that this will be promoted by less argument and more ex- 

 perimental observation. 



That the solution of the underlying question in evolution 

 will still require a long time is evident; as Whitman said 

 in his address before the Congress of Arts and Science in 

 St. Louis in 1904: "The problem of problems in biology 

 to-day, the problem which promises to sweep through the 

 present century as it has the past one, with cumulative inter- 

 est and correspondingly important results, is the one which 

 became the life-work of Charles Darwin, and which can not 

 be better or more simply expressed than in the title of his 

 epoch-making book, The Origin of Species." 



Summary. --The number of points involved in the four 

 theories considered above is likely to be rather confusing, 

 and we may now bring them into close juxtaposition. The 

 salient features of these theories are as follows: 



