HISTORICAL ZOOGEOGRAPHY 



105 



Wide-spread Propor- Propor- Propor- 



forms and tion tion tion Propor- 



uncertain from the from the from tion 



endemic Java Philip- the from 



species bridge pines Moluccas Flores 



Mollusks 30.3% 21.4% 23.3% 15.1% 9.9% 



Reptiles and am- 

 phibians 17.6 37.95 26.4 11.1 6.95 



Birds 28.8 25.6 16.0 19.6 10.0 



Average 25.6 28.3 21.9 15.3 8.9 



It is similarly evident that a part of the fauna of South America, 

 and of the mammalian fauna in particular, is of much later origin than 

 the remainder. The early Tertiary separation of this continent ex- 

 tended through a period of active evolution of the mammals of the 

 northern hemisphere. The older Tertiary rocks of South America con- 

 tain the remains of marsupials, edentates, hystricomorph rodents, and 

 platyrrhine monkeys, but remains of the present South American 

 carnivores, and of tapirs, peccaries, deer, and llamas, are wanting. It 

 is evident that the latter series are later arrivals, and that they have 

 arrived since the reestablishment of the connection with North Amer- 

 ica. An exchange of forms took place at this time, for the armadillos 

 and opossums of southern North America are likewise relatively late 

 arrivals, contemporary with the invasion of the ground sloths and 

 glyptodonts, which reached Pennsylvania. 



Conclusions as to former land connections and former climatic 

 arrangements may thus be based upon zoogeographic data, and if there 

 is abundant palaeontological evidence, such changes and rearrange- 

 ments may be dated with a varying degree of certainty. Zoogeography 

 thus becomes an important aid to geology, specifically to palaeo- 

 geography, and many have regarded this form of distributional study 

 as the capstone of the zoogeographic structure. The value of zoogeog- 

 raphy for the elucidation of former land connections seems to be the 

 higher inasmuch as geologic evidence in this direction is unavailable 

 at the bottom of the sea. F. Sarasin even refers to zoogeography as a 

 form of submarine geology. 20 Most suppositions with regard to former 

 land connections are based on the present distribution of animals. 



The probability and possibility of the conclusions based on these 

 premises must be examined with especial care. Palaeogeographic maps 

 are based on geological and palaeontological as well as zoogeographic 

 evidence, but these three elements are of very unequal value. The 

 geologic evidence of the existence and extent of former seas, which have 

 left their deposits in areas now continental, is most certain. Land con- 

 nections, deduced from the correlation of stratified rocks in areas now 



