288 MENTAL ETOLUTIOX IX ANIMALS. 



pendently arrived at by Mr. Wallace. It is only now tbat 

 Mr. Darwin's Adews upon this subject are published, although 

 they were committed to writing as they appear in the 

 Appendix between twenty and thirty years ago. Mr. Wallace 

 however enunciated substantially the same views in a letter 

 to "Nature" in 1874 (Oct. 8),* from which I shall quote in - 

 c^enso, not only for the purpose of showing the coincidence 

 to which I have alluded, but also because I think that the 

 additional element which Mr. Wallace mentions — i.e., the 

 separation of breeding and subsistence areas — is a most im- 

 portant one. 



" Let us suppose that in any species of migratory bird, 

 breeding can as a rule be only safely accomplished in a given 

 area ; and further, that during a great part of the rest of the 

 year sufficient food cannot be obtained in that area. It will 

 follow that these birds which do not leave the breeding area 

 at the proper season wiU suffer, and ultimately become 

 extinct ; which will also be the fate of those which do not 

 leave the feeding area at the proper time. Now if we sup- 

 pose that the two areas were (for some remote ancestor of the 

 existing species) coincident, but by geological and climatic 

 changes gradually diverged from each other, we can easily 

 understand how the habit of incipient and partial migration 

 at the proper season would at last become hereditary, and 

 so fixed as to be what we term an instinct. It will probably 

 be found that every gradation still exists in various parts of 

 the world, from a complete coincidence to a complete separa- 

 tion of the breeding and subsistence areas ; and when the 

 natural history of a sufficient number of species in all parts 

 of the world 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. 

 The actual causes that determine the exact time, year by 

 year, at which certain species migrate, will of course be diffi- 

 cult to ascertain. I would suggest, however, that they will 

 be found to depend on those climatic changes which most 

 affect the particular species. The change of colour, or the 

 fall of certain leaves; the change to the pupa state of 

 certain insects ; p^^evalent winds or rains ; or even the 



* Captain Hutton also foreshadowed these Tiews in 1872 ; see Trans. 

 New Zealand Inst., p. 235. 



