264 ENVIRONMENT OF VERTEBRATE LIFE, ETC. 



tity. Such an environment would permit an enormous increase in numbers, 

 but the increase in kinds would reach a definite limit, whether large or small. 

 A monotonous environment does not imply a small number of genera and 

 species in a fauna or flora, but it does imply a distinct limit to the number. 

 The struggle for existence can result in the persistence of new forms only 

 so far as the new forms can find an isolation or favorable environment; 

 beyond that point new forms can not survive and a period of stagnation in 

 development will ensue, the stagnation being more or less complete as the 

 monotony of the environment is more or less pronounced and long-continued. 

 This condition will be in the nature of an end-result upon a fauna or flora 

 and the efifect does not necessarily extend to the suppression of variability 

 in the individual organisms. In the struggle for existence, induced by 

 increasing numbers and closer adjustment of the inter-relations between 

 distinct species, the amount of variability might remain the same or even be 

 accentuated. In the author's opinion the tendency to continue the develop- 

 ment along definite lines, call it by what name we will, would be seriously 

 affected by the failure of new forms to develop to maturity. The lack of 

 mature new forms would prevent the survival of new structures demon- 

 strating the tendency of evolution, beyond a certain point, but, at the same 

 time, the constancy of the environmental conditions would tend to fix ever 

 more firmly the tendency and increase the number of variants in that direc- 

 tion. Viewed as a whole, then, the end-result would be, on a large scale, 

 somewhat as discussed in the author's paper on the Linton fauna. The 

 animals in their environment of limited possibilities of morphological expan- 

 sions would soon fill all of the available spaces and new forms would cease 

 to reach maturity, this being largely due to the early extinction of the 

 variants. The continued and increasing pressure to produce new forms 

 because of the fixation of the tendencies would impose upon the fauna as a 

 whole a state of stress to which it would need only the relief afiforded by the 

 possibilities of migration into a new environment or of a change in the 

 environment to find expression in a rapid development of new types. This 

 opinion is opposed by my colleague. Dr. A. F. Shull, who suggests that an 

 inherent tendency to evolution along definite lines would find relief in the 

 mere production of offspring and that the fauna would not experience an 

 increase in any tendency to develop along definite lines or any increase 

 in the number of variants. This suggestion has great force and it may well 

 be that it is the true situation. The author is well aware of the limitations 

 of the evidence at the disposal of the paleobiologist, but the evidence is so 

 conclusive and repeated of long periods of stagnation in evolution followed 

 by rapid development that he can not rid himself of the impression that 

 faunas in periods of stagnation go through a period of preparation, in some 

 form, for their subsequent radiation. 



