MODES OF MAMMALIAN EVOLUTION 647 



made their derivations upon the basis of a Hkeness of sound and 

 meaning in the words compared, so the modern zoologist, 

 in attempting to trace the relationships of animals, must proceed 

 by balancing their similarities and differences of structure. 

 The etymologist had no sure test for distinguishing a true 

 derivation from a plausible but false one, and the zoologist 

 finds himself in the same predicament. How much weight 

 should be allowed to a given likeness and how far it is offset 

 by an accompanying difference, there are no certain means 

 of determining, and we are still in search of those laws of organic 

 change which shall render such service to zoology as Grimm's 

 law did to the study of the Indo-European languages. Doubt- 

 less, the analogy may be pushed still farther, and it may be 

 confidently assumed that, just as sound principles of etymology 

 were established by tracing the changes of words step by step 

 from their modern forms to their ancient origins, so the exist- 

 ing animal forms must be traced back through the inter- 

 mediate gradations to their distant ancestors, before the modes 

 of organic development can be deduced from well-ascertained 

 facts. 



The evolutionary problem has been attacked by the aid 

 of several distinct methods, each of which has its particular 

 advantages and its pecuhar limitations and drawbacks. Most 

 of the methods suffer from the fact that they deal only with 

 the present order of things, and thus resemble the attempt to 

 work out the derivations of languages that have no literature 

 to register their changes. 



(1) Of necessity, the oldest of these methods is Comparative 

 Anatomy, which had made great advances in pre-Darwinian 

 days. It is the indispensable foundation of the whole in- 

 quiry, for an accurate knowledge of Comparative Anatomy is 

 absolutely necessary to the use of the other methods ; in the 

 hands of the great masters it has registered many notable 

 triumphs in determining the mutual relationships of animal 

 groups ; but finality cannot be reached by this method, because 



