272 RECREATIONS OF A NATURALIST 



Granting the hereditary nature of habit now so 

 generally conceded, we have at once conditions for 

 the development of a new instinct, at first doubt- 

 less feeble and uncertain in action, but strengthen- 

 ing by exercise, and by the inevitable "weeding 

 out " of individuals in whom it was weakest. 



With the increased diversity in the conditions 

 of environment called into existence by the great 

 climatic and other changes occurring at or near 

 the close of the Tertiary epoch, there was greater 

 play for the modifying action of physical influences, 

 resulting in the development of new specific types, 

 as well as the instinct of migration. 



Accepting this as the probable origin of migra- 

 tion, we have then to consider how birds are guided 

 in their long journeys. Their ability to find their 

 way across continents and over vast tracts of desert, 

 or sea, is doubtless the result of heredity of habit 

 and the experience of individuals. We are all 

 familiar with the " homing " instinct in carrier 

 pigeons, and with the remarkable way in which 

 lost dogs find their way back to their masters from 

 considerable distances. We have all wondered, too, 

 at the extraordinary way in which birds from a 

 distance will return straight to a well-concealed 

 nest in the middle of a field, marsh, or wood, or in 

 the midst of heather or thick furze. To do this, and 

 to find out an old nesting-place after an interval of 

 many months' absence from the country, implies an 

 exercise of memory as true as it is astonishing. 



Again, the power of vision in birds is very great, 

 far greater than it is in man, as I have been able 



