NEW TROPICAL MILLIPEDS OF THE ORDER MEROCHETA, 

 WITH AN EXAMPLE OF KINETIC EVOLUTION. 



By O. F. Cook, 



Of the United States Department of Agriculture, Washington. 



EVOLUTION OF USELESS CHARACTERS IN MILLIPEDS. 



The adaptive characters of the higher animals and plants, corre- 

 sponding to differences of habits and external conditions of existence, 

 have furnished the principal arguments for the theory that evolu- 

 tionary changes of organisms are brought about by the selective 

 action of the environment. A special evolutionary interest attaches 

 to the structural diversities of lower groups like the millipeds because 

 they enable the theory of selection to be tested by application to large 

 series of biological facts. 



In comparison with the specialized differences of habits, habitats, 

 and food requirements found among the members of the higher classes 

 of plant and animal life, the millipeds may be said to have an almost 

 complete ecological unity. Nearly all the members of the group have 

 essentially the same habits and live in closely similar environments. 

 They pass their lives buried in the humus layer of the soil or among the 

 dead leaves or other decaying vegetable matter that furnishes their 

 food. With such uniformity of external conditions the influences of 

 natural selection should be expected to work in the same direction, so 

 that the structural unity of the group would be preserved. 



The facts do not correspond with this inference from the theory of 

 selection. Elaborate structural specializations have taken place 

 among the millipeds, quite as elaborate as in groups exposed to the 

 selective requirements of specialized external conditions. Darwin 

 and many other writers have argued that evolution would not con- 

 tinue in a group of organisms that remained in a uniform environ- 

 ment. Evolutionary progress in the development of new characters 

 is supposed to be called forth by adaptive response to the stress of 

 external conditions. The theory that selection is the cause of evolu- 

 tion implies that new characters must be useful, but a very large part 

 of the evolution of the millipeds represents the production of charac- 

 ters that seem to be quite useless in any environmental relation. 



Proceedings U. S. National Museum, Vol. 40— No. 1831. 



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