VII. 



THE DISTRIBUTION OF SPECIES. 



IN the present paper I shall consider certain facts 

 of animal distribution as related to the origin of spe- 

 cies. There are many difficulties in 

 Illustrations not bf th f t f geographical distri- 

 arguments. 



bution down to the needs of concrete il- 

 lustration. And in this connection it is especially im- 

 portant to distinguish single illustrations from argu- 

 ments. Isolated cases of geographical variations in 

 species, for example, would not have great value as ar- 

 guments for the development theory were the cases 

 really isolated. The force lies in this fact, that these 

 cases are typical ; that what may be said of one is true 

 of a thousand. In like manner the full force of the 

 laws of homology and heredity can only be felt when 

 their effect is cumulative, as in the mind of the anato- 

 mist who has followed each organ through its protean 

 disguises in a wide range of forms. Again, the force 

 of the argument drawn from embryology does not come 

 from a knowledge of the changes in a single egg. All 

 these studies need the second premise, obtained by years 

 of comparison in different fields of investigation, that 

 no case is isolated. Without this premise, the argument 

 would be incomplete. The few cases of development 

 oj change which can be brought to popular notice are 

 simply illustrations and not proofs. 



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