140 DARWINISM TO-DAY. 



known as individual, fluctuating or continuous variations. 

 He has chosen this name because the single characteristic 

 can change toward but two directions ; that is, toward the 

 plus, or toward the minus direction. In contrast to this 

 kind of variation stand the sudden and discontinuous leap- 

 like changes or mutations which have been for the first 

 time carefully investigated by the praiseworthy labour of 

 de Vries, hitherto having been familiar indeed under the 

 names 'single variations' or 'sports,' but little studied. Con- 

 cerning these linear variations de Vries writes : 'The statis- 

 tical method of the study of variation has now been so 

 generally followed as to make its principles familiar without 

 further discussion, and they may be considered as accepted. 

 The chief principle indicated by the use of the frequency 

 curves is that the characteristics vary in but two directions, 

 that is toward plus or toward minus. The old vague con- 

 ception of an all-sided variation of the single characters 

 has disappeared of its own self.' 



"As highly as I appreciate the great service of de Vries 

 in relation to our knowledge of the suddenly appearing 

 changes, heritable in high degree, I must nevertheless op- 

 pose him in his conclusions touching the selection theory.. 

 In the first place this theory does not reckon alone with 

 linear variations, but also with mutations, if they appear, 

 for it takes the changes as given material without troubling 

 itself about differences in their mode of origin. In the 

 second place it is not correct that a character cannot so 

 change itself through simple addition or reduction that it 

 may not be, in the customary classificatory limits, looked on 

 as a new character. A smooth leaf, a leaf with few small 

 hairs, and one with a thick wool show only linear variations, 

 but in spite of that they may very well serve as character- 

 istic of different species. Nearly related butterflies recall 

 the Vanessas and Lycsenas often show the same funda- 

 mental characters of pattern and form, so that they are dis- 



