174 THE THEORY OF EVOLUTION 



differing vital conditions prevail, deviate from their 

 original specific and generic characters through so- 

 called adaptation to the changed environment. 

 That these deviations are most easily determined in 

 a definite direction on such small islands arises 

 obviously from the necessity of close in-breeding 

 between the few individuals which chance has brought 

 together. 



It can furthermore be established that likewise 

 entire continents, which have been isolated since very 

 long periods (Australia), or only in the younger geological 

 periods, were again connected with others (like South 

 and North America in the Tertiary era), possess quite 

 peculiar animal worlds (like Australia and South America 

 proper) and only on the frontier regions show a mixture 

 (for instance, the extreme North of South America and 

 Central America). 



For Australia the position is considered to be ap- 

 proximately this : Australia became separated when the 

 most primitive forms of the Mammalia first appeared 

 i.e. the Cloaca 1 and marsupial-like forms or, what is 

 much more probable, this continent possessed at the 

 time of its isolation only Marsupials and Cloaca animals 

 and received also no other orders of Mammalia, or again 

 (in the style of the popular doctrine of evolution), no 



1 Cloaca signifies in zoology the common region into which both the 

 urogenital system and the digestive canal open ; all other orders have 

 for both systems separate external openings : ' cloaca ' therefore signifies 

 a trifling degree of differentiation which served and serves as the best 

 criterion for the determination of different grades of perfection. The 

 ant-eater (Echidna) and the Ornithorhynchus belong to this group. 



