178 GENERAL EYOLUTIOK 



111 the history of the growth of the genera Isognomostoma and 

 Triodopsis, the extreme forms of the two series, it is well known 

 tliat at first the coils of the shell are extremely few, as in Binneya ; 

 and that, like it, it is very thin and with a delicately thin edge ; 

 that the turns increase successively in number, as in Vitrina and 

 Hyalina ; and that, finally, the lip thickens, as in Hygromia. Then 

 the umbilicus may close, as in Tachea, or (in Triodopsis) remain 

 open, as in Arionta. In either case a tooth is soon added on the 

 body-whorl (Polymita, Mesodon), and, finally, the full maturity 

 of the shell is seen in the added teeth of the inside of the lip-mar- 

 gin. How many of the stages of the genera Triodopsis and Meso- 

 don are identical with the genera of the series which represent 

 them, I leave to more thorough conchologists, but that some now 

 exhibit and all have once presented illustrations of the relation of 

 exact parallelism, I can not doubt. 



Example 1. — An abundant race of the American deer, Caria- 

 cus virgiJiianus, exists in the Adirondack region of New York, in 

 which the development of the antlers never progresses beyond the 

 spike stage of the second year. Therefore, some individuals of 

 this species belong to Cariacus and some to Subulo. * 



Example 2. — A large part of the individuals of the common 

 snail, Mesodon alhokibris, never develop the tooth of the body- 

 whorl, characteristic of the genus whose definition has to be modi- 

 fied to retain them. 



Example 3. — Many individuals of Triodopsis tridentata from 

 eastern North Carolina occur withotit the lip-teeth, characteristic 

 of the genus Triodopsis. Hence these specimens, though of com- 

 mon origin with others of the species, must be referred to another 

 genus. 



Example 4. — Structural characters are known in many, if not 

 all, species which are said to be *^ inconstant," being present or 

 absent indifferently, thus being useless for definition. They may 

 be rudimental when present or considerably developed. The pres- 

 ence or absence of wings in some species of insects may be cited ; 

 also the presence of generic characters in the male sex of many 

 Coleoptera and their absence in the females. The characters of 



* Since the above was written the facts as stated have been denied. But the 

 author has seen specimens of Cariacus virgmianus and C. maa'oiis in which the 

 horns had assumed the characters of the genus Alces, throwing these individuals 

 into that genus. See "American Naturalist," 1883-'84. (Ed. 1886.) 



