OTHER THEORIES OF SPECIES-FORMING. 233 



known, no species (of birds) has ever been differentiated 

 without the aid of geographical isolation, though evolution 

 may have gone on to an unknown extent ; and, so far as we 

 can judge, geographical isolation must always, sooner or 

 later, be followed by differentiation." And Romanes, him- 

 self, conspicuous as the only pupil and disciple of Darwin 

 personally advised and aided by the master himself, and one 

 of the most brilliant upholders and expositors of Darwin- 

 ism, says :" "Indeed I believe with Mr. Gulick, that in the 

 principle of isolation we have a principle so fundamental 

 and so universal, that even the great principle of natural 

 selection lies less deep and pervades a region of smaller 

 extent. Equalled only in its importance by the two basal 

 principles of heredity and variation, this principle of isola- 

 tion constitutes the third pillar of a tripod on which is reared 

 the whole superstructure of organic evolution." Thus the 

 most ardent believers in the effects of isolation find it, inde- 

 pendent of selection and alone, sufficient to explain species- 

 forming, while the most ardent neglecters of isolation theo- 

 ries find them too slight to be of any consequence at all. We 

 shall take middle ground and find in isolation a factor .of 

 great effectiveness and one wide-spread in its influence in 

 helping to produce the present-day status of the animal 

 kingdom, but yet a factor which shall most fairly be looked 

 on as an auxiliary or helping-theory of natural selection. In 

 fact, to my mind, the proof of the species-establishing effects 

 of isolation, and of the actual existence of isolation (proof 

 of means or modes of isolation), is something much needed 

 by the general natural selection theory for its own sup- 

 port. Selection needs help from isolation. To my mind, 

 also, these means of isolation actually exist, and the result- 

 ing isolation is actually a very potent factor in species-form- 

 ing. The proofs seem to me obvious. 



The name isolation fairly well defines the condition that 

 we are to discuss; (the term segregation has also been used 



