2 6o Habit and Instinct. 



sufficient number of species is thoroughly worked out, we 

 may find every link between species which never leave a 

 restricted area in which they breed and live the whole year 

 round, to those other cases in which the two areas are 

 absolutely separated." 



Here the divergence of the subsistence area from the 

 breeding area may be regarded as an expression of the facts 

 which have to be in some way accounted for; while the 

 hypothesis of natural selection, as applied to the particular 

 case, takes the form of the barest corollary from the theory 

 in general. Those who migrated to and fro survived; those 

 who failed to do so were eliminated. Voila tout ! Unfortu- 

 nately it remains a corollary unverified and unverifiable. 



Mr. Headley quotes * from Mr. Henry Seebohm the 

 cases of the Petchora pipit and the Arctic willow wren 

 which "both winter in the Malay Archipelago. They 

 have extended their breeding grounds from Siberia to 

 Eastern Europe. But though they have moved their 

 summer residence so far west, in winter they still return 

 to their old haunts in the Malay Archipelago, though 

 Africa is more accessible, and, we might imagine, equally 

 suitable." Here we see an extension of range going on 

 under our very eyes, and that in the breeding area which 

 is generally regarded as the more stable and fixed. I do 

 not know whether in these cases the young migrate in 

 advance of the older birds. But in any case it is difficult 

 to say whether this extended range is now instinctive, and, 

 if so, whether it is due to inheritance of the results of 

 acquired experience, or whether natural selection has been 

 the determining cause. In a word, the instinct of migra- 

 tion does not help in the least to solve the general problem 

 of the mode of origin of instincts. 



The determining conditions of the impulse prompting 

 * "The Structure and Life of Birds," pp. 371, 372. 



