CHAPTER VIII 



DIFFERENTIATION 



VV ITH his customary scrupulous fairness, Darwin went out of 

 his way to draw attention to an axiom of taxonomic botany that 

 was seriously opposed to the theory of evolution by adaptation 

 through the agency of natural selection. "Those classes and 

 families which are the least complex in organisation are the most 

 widely distributed, that is to say that they contain a larger pro- 

 portion of widely distributed species. " Incidentally, as the 

 simpler families must upon the whole be the older, this goes a 

 good way towards proving the correctness of the theory of age 

 and area. 



Now upon the theory of natural selection, it is clear that the 

 successful genera must be those that have the largest numbers of 

 species, or the widest distribution, or both; but as they have been 

 developed by adaptive selection, they should surely on the whole 

 be the most complex and specialised, showing the most signs of 

 adaptation. This has always been a difficulty to the supporters of 

 natural selection, and one which has been passed over with little 

 remark. It can be at once explained by the hypothesis brought 

 forward in Age and Area, for upon that the older forms will be 

 the more widespread, and by reason of their age they must be 

 the simpler on the whole, as having been more early formed in the 

 process of evolution. But age and area is incompatible with the 

 theory of natural selection. 



Age and area leads on directly to the theory which Guppy has 

 called Differentiation, though a simpler and better descriptive 

 term might perhaps be found — mutation perhaps, or differential 

 or divergent mutation, for example, if it were admitted that 

 mutations might be large. The essential feature of the theory, 

 originally adumbrated by Geoffroy St Hilaire (41), is that evolu- 

 tionary change goes downwards from the family towards the 

 species, not in the opposite direction. A family begins as a family, 

 and is not graduallv formed bv the destruction of intermediates. 

 At the same time, of course, when it begins it is also a genus and 

 a species, which at the start are all-important to the family; if the 

 species be killed out, the family disappears. As it grows, the single 

 genera and species become less important to it. The name 



WED S 



