General Conclusions. 145 



3. Therefore, and upon the whole, as 

 Darwin so emphatically held, "Natural 

 selection has been the main, but not the 

 exclusive means of modification." 



4. Even if it were true that all species 

 and all specific characters must necessarily 

 owe their origin to natural selection, it 

 would still remain illogical to define the 

 theory of natural selection as indifferently 

 a theory of species or a theory of adapta 

 tions ; for, even upon this erroneous suppo- 

 sition, specific characters and adaptive 

 characters would remain very far indeed 

 from being conterminous — most of the more 

 important adaptations which occur in 

 organic nature being the common property 

 of many species. 



5. In no CASE CAN NATURAL SELECTION HAVE 

 BEEN THE CAUSE OF MUTUAL INFERTILITY 

 BETWEEN ALLIED, OR ANY OTHER, SPECIES—/. <?. OF 

 THE MOST GENERAL OF ALL "SPECIFIC CHAR- 

 ACTERS." 



6. Without Isolation, or the prevention of 



FREE intercrossing, ORGANIC EVOLUTION IS IN 

 NO CASE POSSIBLE. THEREFORE. IT IS ISOLATION 

 THAT has BEEN "THE EXCLUSIVE MEANS OF 



modification," or, more correctly. the uni- 

 versal condition to it. therefore, also, 

 Heredity and Variability being given, the 



WHOLE theory OF ORGANIC EVOLUTION BECOMES 

 A THEORY OF THE CAUSES AND CONDITIONS WHICH 

 LEAD TO ISOLATION. 



IIL L 



