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THE AMERICAN NATURALIST 



[Vol. XLII 



6. The decadent phylum may present as unstable salta- 

 tions, generic or even higher characters of allied dominant 

 groups ; that is a character of generic or even family value 

 in a dominant group may be merely an individual varia- 

 tion in a decadent one. 



7. The members of a dominant group are, ceteris pari- 

 bus, more closely adapted to their environment, their 

 characters less variable, their geographical distribution 

 more restricted. That is, species of dominant groups 

 may be safely based upon less distinctive characters than 

 those of a crescent phylum. 



8. It follows that senility and decadence are the at- 

 tributes of species, families and orders, as well as of the 

 individual. 



9. The older the genus or allied group of species, the 

 more restricted, apparently, is fertile hybridity. For ex- 

 ample : The genus Rhinoceros is an old one that has been 

 but little modified since early Miocene times ; T have never 

 learned of cases of hybridity between living species. 

 Equus arose in early Pliocene times with all its essential 

 modern characters; hybridization between all its living 

 species is not difficult, but the hybrids are infertile. The 

 genus Bos, while beginning in the Pliocene, did not attain 

 full development till Pleistocene times; its numerous spe- 

 cies are continuously fertile in all combinations. Rhi- 

 noceros is long past the zenith of its evolution ; its highest 

 specialization was in the Pliocene, or at most Pleistocene. 

 Equus, too, is past the highest point of its development, 

 perhaps, but not far. Bos, on the other hand, is a domi- 

 nant or crescent type; its maximum specialization is in 

 the present time. 



10. Secondary sexual characters are transmitted to the 

 opposite sex, unless of positive disadvantage. Varietal 

 and specific characters, in the natural course of events, 

 are more or less unisexual at their inception, and the con- 

 stant tendency is for the characters of one parent to be 

 transmitted to offspring of both sexes, even when 

 characters are apparently useless, as seen in the 



