32O GEOLOGICAL BIOLOGY. 



the name Atrypa reticularis and its varieties, there are ob- 

 served certain plications of the surface, of indefinite num- 

 ber, and increasing by bifurcation. The variability or plas- 

 ticity is observed in respect to the rate and extent of the 

 bifurcation, which in the early and middle part of the life- 

 history is indefinite i.e., there is in the species no fixation 

 of the law of this bifurcation ; but gradually there is acquired 

 a tendency to permanency in the two directions of (a) extreme 

 acceleration and of (U) extreme retardation of the rate of the 

 bifurcation in the development of the individual, and the 

 species which may be said to originate by this process, and to 

 be characterized by the different extent of bifurcation attained, 

 are thus gradually perfected (see Fig. 99). In a set of Iowa 

 specimens examined by the author, a well-defined differentia- 

 tion was noted ; the two species are so nearly distinct that it is 

 found, by arranging the forms in order of their resemblances 

 and differences, that there are two well-defined groups, and 

 the intermediate forms, although they almost touch, are so 

 separate that careful study decides for every individual case 

 on which side of the imaginary line it belongs. Thus Atrypa 

 reticularis is an example of very slow evolution. The family 

 characters appeared well defined with the earliest representa- 

 tives of the suborder Helicopegmata; the generic differences 

 were well elaborated at the first stage of the Eosilurian. This 

 species was among the earliest representatives of the genus, 

 and lived nearly as long as we have any trace of the genus. 

 But the great variability or plasticity of certain characters is 

 a peculiar characteristic of the early forms up to mid life of 

 the genus, and might be called a specific character, and the 

 fixation of this variability is very slowly assumed. 



Conclusions Suggested by the Study of Atrypa Reticularis. 

 Natural selection is supposed to result in the fixing of variable 

 characters, and the failure of natural selection to select would 

 naturally result in a continuation of the variability. It is 

 rational to conclude, therefore, that a species' which continues 

 to live without fixing its variable characters is particularly 

 well adapted to live under a wide range of modified conditions. 

 The wide geographical distribution of the species here under 

 consideration confirms this conclusion. That a species does 



