282 STUDIES, SCIENTIFIC AND SOCIAL 



Closely Allied Forms in the Same Area, 



The only other difficulty suggested by Dr. Sclater rather 

 applies to the theory of natural selection itself than to 

 geographical distribution. He asks how we are to account 

 for closely allied forms so frequently inhabiting the same 

 area, while in so many other cases allied species are 

 strictly limited to distinct areas, to the diverging physical 

 and organic conditions of which they are supposed to be 

 adapted. We think that Dr. Sclater has himself furnished 

 a clue to one mode of solution, in his statement that the 

 willow-warbler and the chiff-chaff, though so alike ex- 

 ternally, are yet quite distinct in mode of life and habits. 

 It is evident, that a variation in the habits of a portion of 

 the individuals of a species would lead to their mutual 

 association and at the same time to their separation from the 

 parent form, and would thus obviate that tendency to the 

 intercrossing of the different varieties which would 

 undoubtedly occur if the variation were one of colour or 

 form only. Is it not probable, therefore, that where two 

 or more closely allied species inhabit the same area, they 

 have arisen ot first by means of useful variations of 

 instincts or habits ; while those which inhabit adjacent 

 but separate areas may have arisen by favourable 

 variations of colour, form, or constitution only ? In 

 support of this view it may be noted that the coal and 

 marsh titmice, which are very closely allied, differ con- 

 siderably in habits ; while the great and blue titmice, 

 which are very different in external characters, agree 

 closely in habits, and are often seen together. 



Summary of General Principles. 



Returning to the general question of zoological dis- 

 tribution and its anomalies, it has been shown, I trust, that 

 the only mode of explaining the existing distribution of 

 living things is by a constant reference to those com- 

 paratively slight but often important changes of sea and 

 land, which the most recent researches show to be alone 

 probable; and, what is still more important, by recognising 



