284 THE EVOLUTION TIIEOKY 



that e\'ery isolation of a .species-colony conditions its transmutation 

 to a new species, or, as lias been maintained, first liy Moritz \Vai;-ner, 

 and later bj^ Gulick and by Dixon, that isolation is a necessary pre- 

 liminary to the variation of species — that not selection but isolation 

 alone rendcn's the transmutation of a species possible, and thus admits 

 of its segreyatiou into several ditt'ereut groups of forms. Romanes 

 went so far as to rci^ard the natural selection of Darwin and ^Vallace 

 as a sub-species of isolation, and isolation in its dixerse forms he 

 regarded as the sole factor in the formation of species. He assumed 

 that it was only by the segiegation of individuals whicli did not 

 varj^ that the constant rc\'ersion to the ancestral s[H'cies could 

 be prevented, and he regarded the process of selection as essentially 

 resulting in the ' isolation ' of the fittest through the elimination 

 of the less tit. The idea is correct in so far, that selection undoubtedly 

 aids the favourable variation to conquest over the old forms, precisely 

 because the latter, being less fuN'ourably placed in tlie struggle for 

 existence, are gradually more completely overcome and \\eeded out, 

 so that a constant mingling of the new forms with the old is prevented, 

 just as it is by isolation of locality. Obviously the new and titter 

 forms could not become dominant, could not e\en become permanent, 

 if they were always being mingled again with the old. But whether 

 it serves any useful purpose to bring this under the category of 

 ' isolation,' and to say that mingling with the ancestral form during 

 transmutation is prcNcnted by natural selection, in that favourably 

 varying individuals are isolaled by their superiority from the inferior 

 ones, that is, the non- varying individuals wliich are doomed to 

 elimination, is somewhat doubtful. For my part, I should prefer 

 to retain the original meaning of the word, and to call ' isolation ' the 

 separation of a species-colony by spatial barriers. 



Whether this factor by itself prevents the mingling with the 

 ancestral form as i^tt'ectually as selection tloes, and whether isolation 

 alone and by itself can lead to the evolution of new forms, or })erhaps 

 viust lead to them, must now be investigated. 



I look at this ([uestion from exactly the same point of view as 

 I did nearly thirty years ago, when in a short paper ' I endeavoiu'ed 

 to show that, under favourable cii'cumstances, an indi\idual variation 

 of a species may become the origin of a local variety if it finds itself 

 in an isolated region. Suppose an island had no diurnal butterflies, 

 until one day a fertilized female of a species from the continent was 

 driven thither, found suitable conditions of life, laid its eggs, and 

 became the founder of a colony ; the prevention of constant crossing 



' Uoher ileii Einfluss dcr Isolining mif die ArtbiliUing, Leipzig, 1872. 



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