64 THE BOOK OF MIGRATORY BIRDS 



earth and tree, is at present a mystery, and perhaps ever 

 will be." 



The query as to the causes of the northern or spring 

 exodus has prompted me to make an effort to explain, at 

 least in some measure, those laws which govern migra- 

 tions, not only of birds, but of certain of our mammals 

 as well. It is a subject of which, so far as I am 

 informed, but very little of an explanatory character 

 has been written. Anything which may here be advanced 

 shall be of a tentative nature. The basis of my explana- 

 tions will be the recent theory of Dr. Marsden Manson, as 

 stated in his "Evolution of Climates/' 



This theory has been accepted by many of the advanced 

 scientists of to-day throughout the world. Professor 

 Schaberle, formerly of the University of Michigan, has 

 recently announced that Dr. Manson's theory may be 

 considered as proven. Furthermore, the work of the 

 United States Astro-Physical Observatory, as may be seen 

 in the recently issued report, is favourable to the same 

 conclusion as that reached by Prof. Schaberle. 



This is not the place to state even briefly the principles 

 of Dr. Manson's theory. I may say, however, that it 

 makes clear and logical the various climates which have 

 ruled upon the surface of this globe since life began, and 

 that it has made adherents in the ranks of science from its 

 announcement. Physicists, geologists, and astronomers, 

 these have accepted the theory because of that unity and 

 simplicity which distinguishes it from the many compli- 

 cated and laboured dissertations on the subject which 

 crowd our book-shelves. I shall not say that its acceptance 

 has been universal. That could not be. There are too 

 many who "do not wish to be disturbed" even in the ranks 

 of science to permit such a result. The majority, however, 

 of those who approach the subject in a fair, unprejudiced 

 manner are accepting Dr. Manson's conclusions. 



Almost without exception, however, scientists are 

 agreed that previous to the period termed the glacial or 

 ice age, climates were non-zonal that is, that they were 



