240 THE TRANSFORMATIONS OF THE ANIMAL WORLD 



their geological duration vary very little, it is for 

 the very reason that they are in course of extinction. 

 Thus we note that the duration of existence of 

 phyletic branches is not indefinite, as demanded by 

 the logic of Darwin's hypothesis and maintained 

 by Weismann quite recently. This limitation 

 takes place under the influence of the action of 

 several natural laws, such as the exaggerated size 

 of the body, the over-accentuated hypertrophy or 

 specialization of certain organs, the irreversibility 

 of evolution, and, finally, in a certain measure 

 perhaps, the progressive reduction in variability. 

 It must also be remembered that each phyletic 

 branch goes through a kind of geological course, 

 in which may be discerned a phase of youth, of 

 maturity, and, finally, one of senility or degeneration 

 preparing the extinction of the type. We can 

 henceforward, at least as regards certain groups, 

 commence to define and recognize the characteris- 

 tics of each of these phases. Hyatt has shown 

 that, in the great group of the Nautilidse, each of 

 the branches starts with an infantile stage, in 

 which the successive dwelling-chambers constitute 

 a straight shell or Orthoceraconus ; then we have an 

 adolescent stage, in which the shell is more or less 

 incurved, forming a Cyrtoceraconus or Gyroceraconus ; 

 then an adult stage, in which the coiling of the 

 chambers gives a spiral shell or Nautiloconus ; and, 

 finally, a senile stage, manifested by the uncoiling or 

 asymmetrical coiling, which we have seen above in 

 the case of the Ammonoids. It is only important 

 not^ to ' forget that these stages occur at varying 

 epochs_in each branch, in such a way that we find 



