i64 BENJAMIN KIDD 



Nevertheless it is incredible when we consider that 

 the inherited memory of the birds in their southern 

 home must have continued as a living force or faculty, 

 always ready to display itself in action, for thousands, 

 or tens of thousands, of generations before a change 

 in the climatic conditions made it possible for them 

 to return. 



Perhaps this list of guesses would not be complete 

 without a mention of what may be called the sun 

 theory, pure and simple. It has never to my know- 

 ledge been distinctly stated excepting by Benjamin 

 Kidd in his last and posthumous work, A Philosopher 

 with Nature. He regarded it as original, but the 

 idea is, of course, implicit in all theories of migration. 

 They are built on it. He says: 



It is one of the facts in the migration of birds over which 

 naturaUsts have always found a difficulty, that the migrants 

 both in the eastern and western hemispheres should in their 

 journey to the south often begin to leave their haunts before 

 the food supply in any way fails them, and before they have 

 any physical want known to us indicating a coming change in 

 the conditions of life. But students of the subject have probably 

 not fully reckoned with the deep emotional effect on all wild 

 nature of the waning light in the declining year, and on the 

 uncontrollable instinct to follow the sinking sun begotten in 

 those whose habits of life it affects. 



It is, we see, beautifully simple: the sun is the 

 source of light and heat, which means life; when 

 cold and darkness threaten death as a consequence 

 of its withdrawal, what more natural than that all 

 creatures capable of swift and easy motion should 

 follow it so as to keep alive by keeping near it! At 



