THE DOCTRINE OF EVOLUTION 111 



(born 1861), emphasized the evolutionary importance of dis- 

 continuous variations (mutations) years before de Vries' work 

 appeared. One of the most recent general statements of the 

 principle is to be found in Evolution and Adaptation, by 

 Professor Thomas Hunt Morgan of Columbia University. 

 From his book most of the following statements have been 

 taken. 



According to this principle new species have been pro- 

 duced by sudden and perfectly definite changes (mutations) 

 in the organism, though it is not necessary to assume that 

 these changes are always great. The theory makes no attempt 

 to account for the presence of mutations, but when they 

 occur it is a striking fact that the characters tend to be 

 transmitted to the descendants. De Vries is inclined to think 

 that there are periods of mutation when many and great 

 changes take place, and periods where comparatively little 

 change in the organism occurs. The same mutation may 

 occur time after time and in large numbers of individuals. 

 When a mutation appears its survival will depend on whether 

 it can find a place in nature where it can exist and leave 

 descendants. If the organism is well adapted to its environ- 

 ment, it will leave many descendants ; if it is poorly adapted, 

 it may barely succeed in existing. Useless or even slightly 

 injurious characters may appear, and if they do not too seri- 

 ously affect the perpetuation of the race, they may persist. 

 Since the mutations appear fully formed, there is no diffi- 

 culty in accounting for the early stages of an organ. Thus, 

 on the supposition of natural selection, it is difficult to see, 

 for example, how the first slight movement of the eye of the 

 flounder toward the upper side (see p. 320) could be a favor- 

 able variation, which it should be according to the selection 

 theory, in order to be preserved. Again, a difficulty in the 

 selection theory lies in the fact that the differences between 

 allied species consist largely in differences of unimportant 



