454 PROCEEDINGS OF THE NATIONAL MUSEUM. vol.40. 



in the forms of the segments underneath the earth} 7 covering have 

 produced no apparent external result. The hairs that clothe the 

 surfaces of the segments project into the narrow incisions and bridge 

 them over, so that the adherent layer of earth is not interrupted. 

 The gaps in the margins of tli3 segments are narrower than the deeper 

 parts of the incisions, as if to keep the outlines of the segments and 

 the general form of the body unmodified. The practical result is 

 quite the same as though the margins of the segments had remained 

 entire, as in the related genera. It would be difficult to imagine a 

 more gratuitous evolutionary change, by which the structure of the 

 segments could be so elaborately modified with so little effect upon 

 the external form or environmental relations of the body as a whole. 



It would be rash to insist upon a generalization from any one 

 example of a useless evolutionary change, or even from any one group 

 of animals in which such examples are so abundant as among the 

 millipeds. But if detailed study of many other groups convinces one 

 that the great majority of the differences that distinguish the species 

 and genera are of the same generally useless, nonadaptive character, 

 it becomes impossible to avoid the inference that evolutionary change 

 is not at all limited to the characters subject to the selective action of 

 the natural environment. 



That selection may interfere to retard or forbid the spread of a 

 harmful variation among the members of a species is easy to under- 

 stand, but no concrete explanation has been offered to show how 

 selection can call forth a new character or even bring about any in- 

 creased development of a character already in existence. Selection is 

 able, undoubtedly, to raise the average of expression of any preferred 

 character in a species or other group of organisms, either wild or 

 domesticated, by restricting reproduction to lines of descent in which 

 the preferred character is expressed with the greatest regularity. 

 Yet such an increase in the regularity of expression of a character is 

 not the same as the production of a new character or an increase in 

 the development of a character beyond a previous maximum. If 

 further selection be applied to the progeny of individuals selected for 

 the expression of a certain character, still higher degrees of expression 

 may sometimes be found, but this does not prove that the increased 

 expression represents a new character, or that it is due to selection. 

 The same degree of expression might have been found by wider 

 selection among the members of the parent group. 



The idea that natural selection is the actuating cause of evolu- 

 tionary progress lacks evidence of fact and force of logic. The wide 

 range of diversity found everywhere among the freely interbreeding 

 members of wild species forbids the assumption that the intraspecific 

 differences are all of adaptive value or that the natural tendency 

 is toward a stable, uniform expression of characters. The general 



