326 EVOLUTION AND ANIMAL LIFE 



language, like the spread of a fauna, is limited by natural 

 barriers. It is the work of civilization to break down these 

 barriers as limiting the distribution of civilized man. The 

 dominant languages cross these barriers with the races of man 

 who use them, and with them go the domesticated animals 

 and plants and the weeds and vermin man has brought un- 

 willingly into relations of domination. 



The profitable study of the problems of geographical dis- 

 tribution is possible only on the theory of the derivation of 

 species. If we view all animals and plants as the results of spe- 

 cial creations in the regions assigned to them, we have instead 

 of laws only a jumble of arbitrary and meaningless facts. In 

 our experience with the facts of science w r e have learned that 

 no fact is arbitrary or meaningless. We know no facts which 

 lie beyond the realm of law. We may close with the language 

 of Asa Gray: 



'When we gather into one line the several threads of evidence of 

 this sort we find that they lead in the same direction with the views 

 furnished by other lines of investigation. Slender indeed each thread 

 may be, but they are manifold, and together they bind us firmly to 

 the doctrine of the derivation of species." 



