EXPERIMENTAL EVOLUTION 119 



value of these latter forms of variation, as leading to 

 permanent modifications of structure independently of 

 natural selection. 1 And he goes on to say that, "as my 

 conclusions have lately been much misrepresented, and it 

 has been stated that I attribute the modification of species 

 exclusively to natural selection, I may be permitted to 

 remark that in the first edition of this work (the Origin), 

 and subsequently, I placed in a most conspicuous position 

 namely, at the close of the Introduction the following 

 words: 'I am convinced that natural selection has been 

 the main but not the exclusive means of modification."' 2 

 In the second place, the mutation theory explains away 

 numerous objections to natural selection. It shows how 

 characters that are never of vital importance 3 i.e., matters 

 of actual life or death to a species may arise and be per- 

 petuated. Without mutation this is difficult to explain, 4 



1 Darwin, C. The Origin of Species. 6 Ed. New York. D. Apple- 

 ton & Co., 1902, p. 293. 



2 Darwin almost dispaired of making his position on this point under- 

 stood. The clear statement above quoted, he said, "has been of no 

 avail. Great is the power of steady misrepresentation; but the history 

 of science shows that fortunately this power does not long endure." 

 Darwin. 1. c., p. 293. 



8 As required by Darwin's theory. See quotation above (p. 118), and 

 on p. 97. 



' Other explanations have been offered. For example, sometimes two 

 characters appear to be always associated, so that the presence of one 

 involves the presence of the other; as a mane and maleness in the lion, 

 dicotyledony and exogeny in Angiosperms. The constant association of 

 two characters is often (though not always) due to the fact that the factors 

 for those characters tend to keep together in the same chromosome, in- 

 stead of segregating during the formation of egg-cells and sperms. This 

 tendency is called linkage. The association of smooth (vs. wrinkled) seed 

 with tendrilled (vs. non-tendrilled) leaves in the garden pea, and of red 

 flower-color with round pollen in the sweet pea may be cited as examples of 

 linkage. In such cases one of the characters might be of vital importance 

 to a plant in the struggle for existence and the other not. 



