PLASTICITY AND PERMANENCY OF CHARACTERS. $01 



are long lines of individuals, each running through one or 

 more geological periods, and repeating without noticeable 

 change the precise morphologic characters of its ancestors 

 down to the generic characters, and exhibiting differences only 

 in the specific or less important elements. The specimens 

 exhibiting this law we associate together as a genus and call 

 them by the same generic name, expressive of the fact that 

 they agree in all their morphologic elements, except such as 

 distinguish different species of the genus. The characters of 

 specific value vary during the life-history of the genus, but 

 the generic characters remain unchanged ; or, to apply a spe- 

 cial designation to these two facts, the generic characters are 

 fixed, and the specific characters are more or less plastic. 



Characters which are Plastic at the First or Initial Stage of the 

 Genus. At the initial stage of the genus Spirifer the generic 

 characters may be supposed to have become fixed. The 

 still plastic characters are chiefly seen in a few definite mor- 

 phologic elements. These are : (I) the contour of the shell, or 

 in terms of growth, the relative rate of growth from the nu- 

 cleus outward ; this is seen in specimens with short hinge 

 lines, in others with produced angles at the extremity of the 

 hinge and in the intermediate forms; (II) the vertical extent 

 of hinge area, ranging from low to high area; (III) the del- 

 thyrium, open to closed ; (IV) the surface, evenly arched 

 over, or producing a single median fold, or several folds, ex- 

 tending to the beak, or with intermediate development ; 

 (V) the surface striations, radiating and concentric, fine or 

 coarse, continuous or interrupted, uniform or bifurcating as 

 they develop toward the front. 



The Fixation of Plastic Characters in a Generic Series. It is 

 the various degrees of modification of these characters that con- 

 stitute the specific differences upon which the several species 

 are defined. They are all plastic or variable at the first 

 stage, and the individual species of the genus present certain 

 limits of variation of each of the characters. 



If we go a step farther and classify all the variable char- 

 acters of the genus, we may discover in numerical terms the 

 relation between retention of plasticity and the passage of 

 time, or the effect of time in limiting the variability of the 



