THE MIGRATION OF ANIMALS. 125 



introduction to the question, "How have migratory habits been 

 inaugurated and perpetuated in birds?" It is needless to say that 

 any answer which can be given to this query must be speculative in 

 its nature. No direct evidence of the beginning of this habit in any 

 animal is at hand, nor, from the very nature of the case, can such 

 testimony be procurable. Hence we have to correlate facts, to 

 marshal them in relation to one another, and to string them together 

 by aid of generalisation and theory. Such is the true relation of theory 

 to fact a relationship which not only permits but demands, firstly, 

 the correspondence of facts and their connecting hypothesis ; and, 

 secondly, the ability and desire to modify the theory according as 

 new facts or higher interpretations dawn upon us. One or two 

 features in the case of birds seem in some degree to aid us in form- 

 ing a natural theory of migration. This habit, it should be remem- 

 bered, occurs in very varied and different groups of birds. Species, 

 genera, and families widely separated in structure, food, and habits, 

 exhibit the like instinct of periodically passing from one country to 

 another at certain seasons. Through such a fact the zoologist points 

 out that migration is an acquired habit, and not one originally or 

 from the first affecting uniformly great groups or large classes of 

 birds. 



The observation that widely separated birds exhibit the same 

 habit further warrants the inference that the varied species have 

 acquired migratory habits through exposure to like conditions. Now, 

 what were these "conditions"? Suppose that, as in America, a 

 species of bird was presented with a continuous land surface running 

 north and south. Such a bird, subjected, it might be, to increasing 

 cold from the north, would pass easily and readily southwards. An 

 alteration of the temperature in favour of a more genial climate, and 

 the retreat of the cold, would be followed, on the other hand, by the 

 northward return of the birds. If we suppose the bird to have been 

 an insect-feeder, the case is presented still more feasibly to view, 

 inasmuch as the failure of the food supply from cold, and its revival 

 during the returning heat and geniality of climate, would constitute a 

 sufficiently powerful incentive to migrate southwards, and an equally 

 powerful inducement to the northward return. But is this case of 

 alternation of hot and cold epochs, or of cold with genial climates y 

 anything more than supposition ? The geologist's reply bears that 

 in comparatively " recent " times, and in the Miocene period, Europe, 

 and the northern parts of the world generally, possessed a climate 

 which, if not exactly tropical, was the reverse of rigorous. Succeed- 

 ing the genial Miocene epoch, with its subtropical flora and fauna, 

 the great ice age slowly but surely dawned, blighting the plants 

 which had formerly flourished in plenty beneath a kindly sun, cover- 

 ing hill and dale with a great ice-sheet, and filling the valleys with its 



