-EVOLUTION AND DISTRIBUTION OF ANIMALS. 315 



to tlieir environment — an adjustment which is made more and 

 more complete by the ceaseless destruction of the unadjusted. 

 According to this theory, the * same causes which have produced 

 difference of species in the past must be still at work, and must 

 continue to produce similar differences in the future. To the 

 theory of derivation is opposed the old idea of " special creation." 

 But this theory of special creation has never had in science other 

 than a provisional existence. It was a mere name for a process 

 not understood. If each of the millions of species of animals and 

 plants living and extinct came about by a " special creation," then 

 special creation can not be an operation outside the limits of law. 

 It is simply the name given in ignorance to the law by which 

 species are produced. What has been done so many times must 

 be done in some uniform way. What this way is, the theory of 

 evolution professes in some degree to define. 



The fact is, the theory of development gives the only clew by 

 which the naturalist can be guided in his work. If the mutual 

 affinities of species do not depend on the law of heredity, they are 

 unintelligible. They are impossible. If the variation of species 

 is really immutability in disguise, we can not trust our senses. 

 We are left to choose between some form of the development 

 theory and a hopeless unscientific agnosticism, content with the 

 surface facts, and ignorant of the laws of which these facts are 

 the expression. 



I do not wish to-night to discuss either the general question of 

 evolution nor that special theory of the method of evolution 

 which is associated with the name of the master of modern zool- 

 ogy. I shall take evolution and Darwinism for granted, and con- 

 fine myself to a statement of certain facts and principles in the 

 science of zoogeography and to their bearing on the question of 

 the origin of species. There are many difficulties in bringing the 

 facts of this science down to the needs of concrete illustration. 

 A science so broad as to include all human history at once with 

 the history of every group of animated organisms can not well be 

 compressed into a discussion of a single hour. And with this I 

 may recall the additional difficulty, present in all discussions of 

 the subject of evolution, of distinguishing single illustrations 

 from arguments. Isolated cases of geographical variations in 

 species would not have great value as arguments for the develop- 

 ment 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 he- 

 redity can only be felt when their effect is cumulative, as in the 

 mind of the anatomist who has followed each organ through its 

 protean disguises in a wide range of forms. 



