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THE AMERICAN. NATURALIST [Vol. XL V 



bodies of water are all impassable to most land animals, 

 "We can not then wonder that the fauna of Xortli Africa 

 is sharply differentiated from that which lies to the 

 south of the Sahara; and so also that the Himalayas 

 form a boundary between two zoogeographic kingdoms. 



How, now, are we to explain the cases where two simi- 

 lar faunas are separated from each other by what seems 

 to be a similarly impassable harrier? How is it pos- 

 sible, to draw an example from our own archipelago, to 

 make comprehensible the fundamental similarity of the 

 fauna of Sumatra and of the ^Ialay Peninsula? In this 

 case only two possibilities are thinkable: either by some 

 means or other the animals have been able to get across 

 the sea, by flying or swimming, by the aid of wind or 

 drift-wood, or through transport by human agency; or 

 else there has been an earlier land connection which has 

 now completely disappeared. 



Above all others this last mentioned explanation is the 

 most fruitful for further investigation, as in general it 

 involves calling to aid geologic factors to elucidate the 

 reason for zoogeographic evidences of differentiation. 

 To Wallace belongs the credit of having brought to light 

 the true import of these factors. Zoogeography, then, 

 may now be considered as a science auxiliary to geology. 



It is evident after stating the foregoing premises that 

 it is quite impossible to divide up the earth into sharply 

 defined areas of distribution which hold alike for all 

 groups of animals. Different groups owe their spread 

 over the earth to different reasons. Some may pass 

 easily over mountains; others (notablv birds) may as 

 easily cross the sea; some are far more dependent upon 

 climate and the condition of the ground on which they 

 exist than others; the oldest groups of animals, speaking 

 geologically, have had far more time to distribute them- 

 selves than have the younger; etc. We must, however, 

 confess that, following in the footprints of Wallace, per- 

 verted conceptions have long held sway regarding the 

 Indian archipelago. 



