296 MENTAL EVOLUTION IN ANIMALS. 



be pursued. It is, without question, an astonishing fact that 

 a young cuckoo should be prompted to leave its foster-parents 

 at a particular season of the year, and without any guide to 

 show the course previously taken by its own parents ; but 

 , this is a fact which must be met by any theory of instinct 

 [l which aims at being complete. Now upon our own theory it 

 can only be met by taking it to be due to inherited memory. I 

 confess to me it seems incredible that many hundred miles of 

 landscape scenery should constitute an object of inherited 

 memory,* to say nothing of long stretches of ocean ; but the 

 case is not quite so hopeless as to require so extreme a 

 hypothesis. When we say that upon our theory the young 

 cuckoo must be supposed to find its way on its first journey 

 by inherited memory, we need not necessarily affirm that this 

 is the memory of a landscape. As I have said in the pre- 

 vious paragraphs, we do not yet know what it is that guides 

 the course of migratory birds in general ; but whatever this 

 may be, it can scarcely be the appearance of the country over 

 which they pass, seeing not only that the distances are so 

 great and that two hundred or three hundred miles of ocean 

 may separate one piece of country over which they travel 

 from another, but also that the journeys may be taken by 

 night. Of what, then, is the inherited memory on which the 

 young cuckoo (if not also other migratory birds) depends ? 

 j We can only answer. Of the same (whatever this may be) as 

 ' that upon which the old birds depend. When we certainly 

 know what this is, we shall first be able to ascertain whether 

 it is incompatible with the theory of evolution to suppose 

 that it can be an object of hereditary memory. Thus, for the 

 sake of example, let us suppose that the old birds in their 

 outgoing journey guide their way by flying towards the south 

 wind (as has been suggested to me by Mr. William Black, 

 who believes that swallows always start against the south 

 wind); heredity would in this case have an easy task in 

 associating the warm moist breath of this wind with a desire 

 to fly against it. Of course I only adduce this suggestion in 

 order to show how simple the mere question of heredity 

 might become, if once we knew the means whereby migratory 

 birds in general find their way. The only difference between 

 the faculty of homing and the instinct of migration, so far as 



* This theory was first advanced bj Canon Kingsley {Nature, Jan. 18, 

 18G7), and has since been independently suggested by several writers. 



