288 THE BIOLOGY OF THE SEASONS 



instinct, part and parcel of the bird's inherited constitution, 

 and doubtless varying in precision and in content in different 

 birds. But it is not inconsistent with this to believe that 

 keen senses, quick perceptions, good memory, obedient 

 following of leaders, and a sense of direction, are also of 

 great importance. We have ourselves so few instincts 

 that it is difficult for us to get mentally near creatures in 

 whose life instinctive activity bulks largely, but we need 

 not suppose that the migratory instinct is isolated from the 

 bird's highly evolved sensory life, or from its undoubtedly 

 good memory. 



If we start right away with the astonishing migrational 

 movements of the knot or the curlew-sandpiper, and assert 

 that these have an instinctive basis, we may be accused 

 of escaping from a formidable difficulty under the shelter of 

 a word. But in evolutionary interpretation this impression 

 always arises when we begin by thinking of the most ela- 

 borate and finished product, instead of the simplest possible 

 illustrations. If we are thinking of the problem of the 

 evolution of the eye, we should not begin with man's or 

 the eagle's ; if we are thinking of the problem of the 

 evolution of sex, we should not begin with the elaborately 

 finished dimorphism of peacock and peahen, but with cases 

 like sea-urchins, where it requires a microscope to tell the 

 male from the female. So in regard to migration, we may 

 reasonably begin with the short migratory movements, say 

 of blackbirds, which are relatively simple affairs. 



If we are convinced that there is warrant for speaking 

 of a migratory instinct, we may pass to the next question, 

 What conditions led to the establishment of this instinct? 

 When we think back imaginatively to the beginning of 

 migration, we see the emergence of a new idea, which is 

 called, in the technical language of Biology, a mutation. 

 A new constitutional type arose the revolutionary, who 



