SPECIES AND THEIR ORIGIN 299 



have produced others always like themselves." 1 

 This conception of the fixity and permanence of 

 species was not always held so firmly as it was during 

 the eighteenth and the first half of the nineteenth 

 centuries. Milton and the type of mind of which 

 he was the spokesman doubtless had much to do 

 with riveting this idea, essentially a theological 

 dogma, upon the popular mind. 



Any branch of natural science has its beginning 

 in the classification and assembling of data. The 

 number and diversity of forms of plant and animal 

 life have proved so great that until quite recently 

 the energies of naturalists were almost exclusively 

 directed toward the classifying and naming of 

 species. This has been done largely on the basis 

 of " outward form," and the methods of the scientist 

 are not different in kind from those of the non- 

 biologist. If, for example, the latter should be 

 given a basket of fishes of all sorts, he would have little 

 difficulty in picking out by sight the various kinds, 

 and he probably would make but few mistakes. If 

 the basket should contain exclusively fresh-water 

 fishes, it might be considerably more difficult to 

 sort accurately the various kinds. One's ability to 

 do so would depend a good deal upon his quickness 

 of eye in appreciating details of structure. These 

 details are the same ones that have already been 

 denoted " characters " in another connection. Con- 



1 " Species tot sunt, quot diversas formas, ab initio produxit infinitum 

 Ens, quce formce secundum generationis inditas leges produxere plures, 

 at sibi semper similis." 



