THE METHOD OF CREATION OF ORGANIC FORMS. 177 



never develop it except in abnormal cases in the most vigorous 

 maturity of the most northern Cariacus {O. virginianus), while 

 the South American Subulo retains to adult age the simple horn 

 or spike of the second year of all CervidcB. 



Among the higher Cervidm, Eusa and Axis never assume char- 

 acters beyond an equivalent of the fourth year of Cervus. In 

 Dama the characters are, on the other hand, assumed more rapidly 

 than in Cervus, its third year corresponding to the fourth of the 

 latter, and the development in after years of a broad plate of bone, 

 with points being substituted for the addition of the correspond- 

 ing snags, thus commencing another series which terminates in 

 the great fossil elk, Megacerus. 



Eeturning to the American deer, we have Blastocerus, whose 

 antlers are identical with the fourth year of Cariacus. Corre- 

 sponding with the Dama-Megacerus type of the Old World, we 

 have the moose (Alces) developing the same palmate horn on the 

 basis of Cariacus (i. e., without eye-snag). 



Example 2. — I select the following series, embracing the ma- 

 jority of the genera of the North American Helicidse.* 



1. Turns of spire very few ; wide umbilicus ; shell thin, with thin lips, .^iwne^a. 



2. Turns few, but more ; rest as above Vitrina, 



3. Turns still more numerous ; rest as above Hyalina. 



4. As No. 3, but lip thickened inside Hygromia. 



5. Coiled ; umbilictis closed ; lip thickened inside and out . . Taehea and Pomatia. 



6. Same, with a parietal tooth Mesodon. 



7. Same, with parietal and two interior lip teeth Isognomostoma. 



* * Recommencing at Xo. 4. All with open umbilicus. 



5. As No. 4, but lip thickened in and out Arionta. 



6. Same as No. 5, but with parietal tooth , Polymita. 



1. Same, with both parietal and lip teeth Triodopsis. 



The successional relation of these genera may be represented 

 in such a diagram as this : 



Umbilicus open. Umbilicus closed. 



rj * * 



6 * * 



5 « * 



4 * 



3 * 



2 * 



1 * 



* See Tryon, " Terrestrial Mollusca of the United States." Probably other (e. g., 

 dental) characters distinguish some of these genera, but the above furnishes the 

 history of one set of characters. 

 12 



