OBSERVATIONS ON MIGRATION. 13 



Thus, with regard to food requirements, it is obvious that, 

 when birds of a given species are found permanently inhabit- 

 ing a certain area at all seasons, the natural or climatic 

 conditions of that area do not render migration imperative. 

 Therefore, when it is seen that large numbers of such birds 

 do migrate, and traverse perhaps great distances, it is clear 

 that any hypotheses based on considerations of temperature, 

 climatic conditions, or the like, must be abandoned. Possibly 

 such movements may be dictated by the quantity (rather 

 than by the quality) of the food- supply ; for, though a tract of 

 country may be well adapted for and inhabited by a certain 

 number of such birds, yet if an enormous additional influx 

 of foreigners be suddenly thrown upon it, the area becomes 

 overstocked, its resources unequal to the increased demands 

 upon them, and a proportionate exodus or redistribution of 

 population necessarily takes place. 



All these, however, and cognate circumstances can only be 

 regarded as regulating factors they are not the first cause 

 of migration ; and there are, moreover, many cases in which 

 no such factors appear to operate. 



Many theories in explanation of the migratory instinct 

 have been advanced by speculative naturalists. Some of 

 these are, at any rate, very ingenious ; but, resting on no 

 solid basis, partake more of a poetic than of a scientific 

 character. Indeed, this erection of highly imaginative 

 hypotheses, in support of which it is so easy to collate a 

 quantity of what looks like circumstantial evidence, but 

 which are so incapable of direct proof, is of dubious utility. 

 No doubt causes can be assigned to every effect, a reason to 

 every fact ; but it is perhaps wiser to admit that, with our 

 finite knowledge, there yet remain things which cannot be 

 explained. 



One theory, however, has always appeared to me to stand 

 on somewhat more tangible foundations than the rest ; and, 

 though it may be at the risk of appearing to neglect in 

 practice what I have just preached, I will venture very 

 briefly to refer to it. 



The theory I allude to is that of the Polar origin of all 

 life. This subject is now being discussed by Dr. Tristram 



