January 8, 1904.] 



SCIENCE 



75 



The first assumption, of the origin of the 

 perfect leaf pattern by a single mutation, is 

 unsupported by evidence and to me seems very 

 improbable. That the resemblance arose by 

 the cumulation of a series of mutations inde- 

 pendent of selection seems no less improbable, 

 for in this case we have either to assume some 

 mysterious internal regulation of the rauta- 

 tions directing them all in one direction, or 

 else we must assume that among the many 

 possible mutations only those that were in 

 the direction of closer imitation happened to 

 occur. The latter is of course practically 

 impossible upon the theory of probabilities 

 and the former leads us into a realm of 

 darkness which we seem at present unable 

 to explore. If, however, there is reason to 

 believe in such internal directive influence, we 

 are not justified in rejecting it because of our 

 inability to study its nature and action. I 

 can not see that we have such evidence. 



I have been impressed with the feeling that 

 Professor Morgan has allowed his opposition 

 to Darwin's conception of evolution by the 

 selection of favorable ' fluctuating variations ' 

 to cause him to understate the importance of 

 selection, though in parts of his book he recog- 

 nizes that selection acts on mutants and va- 

 riants. The Darwinian theory and the theory 

 of evolution by selection are not identical, yet 

 Professor Morgan frequently refers to them 

 as if they were so. If mutations be distinct 

 from fluctuating variations, as our as yet very 

 scanty evidence seems to suggest may be the 

 ease, still both mutations and variations, so 

 far as we can see, would be subject to selec- 

 tion. The theory of selection is an explana- 

 tion of some of the phenomena of adaptation. 

 It is difiicult to see that the mutation theory, 

 apart from selection, aids us in understanding 

 or imagining how this adaptation, the most 

 general phenomenon in organisms, has been 

 secured. 



Mutation may be the mode of origin of 

 certain useful qualities, but it is difiicult to 

 see how it explains their retention and per- 

 fection. The theory of selection makes no 

 pretense to explain the origin of varieties or 

 mutations. It attempts to explain the adap- 

 tation of organisms to their conditions of life. 



such adaptation resulting from the selection 

 of those individuals which vary or mutate in 

 useful directions. The theory of selection 

 begins where the theory of mutation leaves off. 



Not even a combination of DeVries's muta- 

 tion theory with Weismann's theory of germ- 

 inal selection would give us, without natural 

 selection, an explanation of progressive per- 

 fection of adaptation. We should still need 

 to add ISTageli's, or rather St. George Mivart's, 

 perfecting principle. 



The work of DeVries seems especially val- 

 uable since it brings to the front such ques- 

 tions as the following: 



Are there mutations which are distinct from 

 fluctuating variations ? Are fluctuating varia- 

 tions restricted to rather narrow limits, and 

 are the larger variations which occur of a dif- 

 ferent sort, establishing a new mean about 

 which a new series of fluctuating variations 

 cluster ? 



Are mutations (or variations) definite or 

 indefinite? Do they follow certain lines or 

 do they occur in all directions? 



If the direction of mutations (or variations) 

 is wholly or in part predetermined, what are 

 these predetermining factors? Are they in- 

 ternal (involved in the nature of the organ- 

 ism), or external (environmental), or both? 



Is there a tendency in mutants (or variants) 

 to revert toward the condition of the parent 

 stock? 



Are mutants (or variants) of one sort more 

 (or less) fertile or more (or less) vigorous 

 when bred together than when bred with the 

 parent stock or with mutants (or variants) of 

 another sort? Does mutation (or variation) 

 cause partial (or complete) segregation? 



Are hybrids between mutants (or variants) 

 of different sorts or between mutants (or 

 variants) and the parent stock intermediate 

 in character between the two parents, or do 

 they follow wholly or chiefly one parent? If 

 the latter, which parent is followed in the 

 several kinds of crosses? 



Upon most of these points the observations 

 of DeVries have an important bearing, though, 

 without much further observation, they do not 

 decide them. 



It seems possible that one of the most im- 



