268 ENVIRONMENT OF VERTEBRATE LIFE, ETC. 



variants upon the strongly impressed homoplastic characters were now able 

 to survive and in the multitude of new forms far-reaching readjustments 

 were necessary to the changed interrelationships. The environment, long 

 monotonous, became increasingly diversified, until the elevation and the 

 climatic change had produced their full effect, and new avenues of migration 

 were constantly opening westward. The number of individuals was far 

 less relative to the available space, and the cloture of the environment was 

 removed. In the readjustment many of the animals found an isolation 

 which permitted rapid development of morphological peculiarities. 



The author must here repeat to some extent his conception of isolation 

 in the sense here used. The environment of any organism is the sum of 

 all its contacts with the external world, organic and inorganic. If the 

 organism, by virtue of its structure, habits, acquired immunity, or other 

 means attains a position in its enviroment where it is relieved from prejudicial 

 contacts either in part or in toto, it just so far attains an environmental 

 isolation and is free to develop individual peculiarities. This is decidedly 

 different from geographical isolation, though the latter may result in the 

 same benefits to the organism. Environmental isolation may be attained 

 even in a very thickly inhabited region. A typical case is that of the 

 modern skunk; another is the Permo-Carboniferous reptile Dimetrodon, 

 which, by increase in size, agility, and raptorial powers, so far dominated the 

 fauna in which it lived that it was isolated from many disadvantageous 

 contacts and developed probably incipiently useful structures to marked 

 excess. 



Such a condition of environmental isolation, attained by only a portion 

 of the fauna, accompanied by close adaptations in the interrelationships 

 of the members of the fauna, could only be attained after a really long period 

 of association; in the early stages of such an association, changes of far- 

 reaching effect would occur. The individuals would be free in a large 

 measure from peculiarities, except inherited ones developed under the earlier, 

 more stable conditions. Such peculiarities would be very apt to be dis- 

 advantageous, and the animals possessing them would disappear, and the 

 less-specialized forms would gradually assume peculiarities as adjustment 

 took place under the new conditions. As intimated above, the extinction 

 of certain forms and much of the readjustment would take place in a time 

 of rapid evolution and the record would be very faulty from the lack of 

 preserved material. It has frequently been shown that the changes from 

 uniform conditions of the land to elevated and disturbed conditions have 

 always been relatively rapid, while the return to low lands and a positive 

 movement of the strand-line have been relatively slow. Thus the air- 

 breathing life was uniformly subjected to relatively rapid changes effecting 

 a violent disturbance of the established relations, followed by a long period 

 of extremely gradual return to monotonous conditions, producing frequently 

 a static environment, during which readjustment took place. 



