no. 1831. NEW TROPICAL MILLIPEDS—COOK. 455 



facts of diversity and interbreeding support the opposite view that 

 evolutionary change of characters in species is a spontaneous, kinetic 

 process, independent of the selective action of the environment. 1 



Recognizing evolution as an antecedent fact, the influence of 

 natural selection can be understood, for it represents the power of 

 the environment to determine the directions that evolutionary 

 progress may take. It is plain that natural selection must favor the 

 expression of characters that prove useful, and forbid or restrict the 

 expression of those that prove harmful. That such a regulation of 

 the characters of a species by the standards of the environment may 

 profoundly affect the subsequent course of evolution is also easy to 

 understand, without supposing that selection actuates the progres- 

 sive development of the new characters. 



The causes of evolution are to be sought, not in the environment, 

 but in the organization of species as groups of individually diverse, 

 freely interbreeding organisms. The results of the evolutionary 

 process, as shown in such groups as the millipeds, indicate that varia- 

 tions not only occur but become established in expression as new 

 characters without having any direct adaptive value for selection to 

 work upon. Thousands of differences between species, genera, 

 families, and orders have come into existence while the environment 

 of the group as a whole has remained practically unchanged. Instead 

 of evolution being limited to the adaptive characters that are fos- 

 tered by selection there seems to be full liberty of change in all 

 directions that are not too harmful to the environmental interests 



of the species. 



A NEW GENUS FROM PORTO RICO. 



The following is a formal description of the animal discussed in 

 the preceding section: 



IOMUS, new genus. 



Type. — Iomus incisus, new species, from Porto Rico. 



Diagnosis. — Related to Tridesmus Cook, also from Porto Rico, 

 and to Docodes7nus Cook from St. Vincent, but with the margins of 

 the carinee deeply incised, the last segment reduced and concealed, 

 the broad trilobed apex exceeded by large subclavate dorsal processes 

 of the penultimate segment. 



Description. — Body small, oblong, abruptly rounded at the ends, 

 about four times as long as broad; dorsum rather strongly convex, 

 the carinas depressed nearly in the direction of the dorsal arch. 

 Dorsum with four longitudinal rows of dorsal tubercles, enlarged 

 into subclavate processes on posterior segments. 



Head concealed and compressed under the expanded first segment, 

 facing ventrad; vertex covered to the level of the antennae with a 



1 0. F. Cook, Methods and Causes of Evolution, Bull. 136 Bureau of Plant Industry, U. S. Department 

 of Agriculture, 1908. 



