27O COOK 



may exist in a species at any particular period would lead, after 

 all, to no truly progressive change. Nothing is gained for evo- 

 lutionary purposes by attempting to explain new characters 

 merely as reversions or as new combinations. 



Nor can such assumptions fully account for the facts, since 

 it is often obvious that absolutely new and unprecedented evo- 

 lutionary departures sometimes appear, which could not be 

 accounted for by any combination of characters existing in the 

 remaining members of the group. Such are the remarkable 

 crests developed on a few of the anterior segments of East 

 African millipedes of the family Oxydesmidae, specialized 

 structures which are entirely without analogy in the remainder 

 of the order Merocheta or, for that matter, of the entire class 

 Diplopoda. 



It would be altogether presumptuous, of course, to insist that 

 any particular variation or mutation represented the very first 

 appearance of its type in the history of the species. It is usual 

 to ascribe variations to possible admixtures of blood at some 

 point in the genealogy of the individual, near or remote. But 

 these suggestions, even if justified for particular cases, should 

 not be allowed to obscure the more fundamental consideration 

 that the very idea of a progressive evolution implies the origina- 

 tion and development of new characters, both of form and of 

 structure, and the opening of new environmental relations for 

 the species. 



Of the causes of new characters we are, as yet, in ignorance, 

 but of their uses we need be in no doubt. New characters not 

 only make evolution possible, but by true symbasic interbreed- 

 ing they help to maintain the vitality or organic efficiency of the 

 species. Neism reinforces heterism and contributes to evolu- 

 tionary progress. New characters are not averaged away and 

 obliterated by interbreeding, but are prepotent. They tend to 

 spread throughout the species and to become more and more 

 accentuated. 



That variation may bring an increase of the vegetative vigor 

 or vital efficiency of the organism could not be more clearly 

 shown than in the numerous instances where unusual bodily 

 strength and hardiness accompany reproductive debility or even 



