5 8 PRIMARY FACTORS OF ORGANIC EVOLUTION. 



gions of the Great Basin and Colorado Desert. On 

 the Pacific Slope, north of California, the color again 

 increases, with a tendency to heavy, sombre tints over 

 the rainy, heavily-wooded region of the Northwest 

 Coast." 1 





2. VARIATION IN STRUCTURAL CHARACTERS. 



Modifications of structural characters may appear 

 quite independently of variation of specific ones. In- 

 deed, generic characters have at times changed 

 completely without the appearance of corresponding 

 changes in the more superficial characters which de- 

 fine the species. Thus changes in the dentition of 

 some of the Mammalia appear within the limits of 

 species, which, should they become permanent, would 

 entitle the two sets of individuals which display the 

 different dentitions to be placed in different genera. 



Some striking examples of how generic characters 

 may undergo metamorphosis without corresponding 

 changes in specific characters, have been brought to 

 light by Dr. William H. Dall among the Brachiopo- 

 dous Mollusca. Some of the species of different gen- 

 era can scarcely be distinguished, except by compari- 

 son of their generic characters. I have cited the 

 axolotls as illustrative of this phenomenon. Here the 

 same species may reproduce as a permanent larva, or 

 as an adult. Dume'ril has shown that the North Amer- 

 ' ican salamander (Amblystoma tigriumn} can lay and 

 fertilize eggs before the metamorphosis is passed, 

 have since observed that the females of the allied spe- 

 cies of Amblystomidse, the Chondrotus tenebrosus B. and 

 G., of California contain mature eggs ready for de- 



1 The Radical Review, May, 1877. 



