THE THEORY OF MUTATIONS 97 



is this last term. " By the postulate of common 

 descent," he continues, " we take it that the first 

 term differed widely from the last, which never- 

 theless is its lineal descendant ; how then was the 

 transition from the first term to the last term 

 effected ? If the whole series were before us, 

 should we find that this transition had been 

 brought about by very minute and insensible 

 differences between successive terms in the series ? 

 or should we find distinct and palpable gaps in 

 the series ? In proportion as the transition from 

 term to term is minimal and imperceptible, we 

 may speak of the series as being continuous ; while 

 in proportion as there appear in it lacunae, filled 

 by no transitional form, we may describe it as dis- 

 continuous" (p. 15). Continuous or discontinuous ? 

 That is the question around which the present 

 article turns. 



We have already seen how Darwin answered 

 the question, and that Mivart doubted the correct- 

 ness of his conclusion. De Vries' work, with which 

 we are here concerned, is devoted to the main- 

 tenance of the thesis of discontinuity. For, accord- 

 ing to his view, fluctuations, that is to say the 

 smaller variations which are constantly going on 

 within a species, the minute differences between 

 parents and children, the characters which dis- 

 tinguish one child from another these things 

 never do andfnever can produce a new species, 

 even a new elementary species. " Fluctuations 

 are linear, amplifying or lessening the existing 

 qualities, but not really changing their nature. 

 They are not observed to produce anything quite 



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