DARWIN AND MULTIPLE ORIGINS 251 



have prevented the independent origin of other 



mammals. To this argument Darwin replied : 



'I know of no rodents on oceanic islands (except my 

 Galapagos mouse, whicli may have been introduced by man) 

 keeping down the development of other classes. Still mw^ 

 more weight I should attribute to there being now, neither 

 in islands nor elsewhere, [any] known animals of a grade of 

 organisation intermediate between mammals, fish, reptiles, 

 &c., whence a new mammal could be developed. If every 

 vertebrate were destroyed throughout the world, except our 

 now well-established reptiles, millions of ages might elapse 

 before reptiles could become highly developed on a scale 

 equal to mammals ; and, on the principle of inheritance, 

 they would make some quite nmv class, and not mammals ; 

 though possibly more intellectual ! ' ' 



Many years later, in a letter to the Duke of 

 Argyll (September 23, 1878), Darwin gave a 

 more complete answer to the extreme suppoi"ters 

 of the hypothesis of multiple origins, at the same 

 time refuting the opinion — not uncommon even 

 at the present day— that a terrestrial species such 

 as man may exist on Mars or on some other 

 body outside the earth. For Darwin shows in 

 the following letter that, in order to produce the 

 same species twice over, the same material must 

 have been subject to the same selection at every 

 stage, right back to the unknown starting-point 

 of organic evolution. 



' As far as I can judge, the improbability is extreme that 

 the same well-characterised species should be produced in 

 two distinct coimtries, or at two distinct times. It is 

 certain that the same variation may arise in two distinct 

 places, as with albinism or with the nectarine on peach-trees. 



' Sept. 23, 1860. Li/e and Letters, ii. 344. 



