204 Canon Tristram on the Polar Oritjin of Life. 



124. [B., P.] BuTORinEs javanica. [E., W.] 



1.25. [B., P.] GORSACHIUS MELANOLOPHUS. [W.] 



126. [B., p.] Sterna bergii. [W.] 



127. Sterna sinensis. [W.] 



128. [B., P.] Hydrochelidon hybrida. [W.J 



129. Fregata minor. [W.] 



XV. — The Polar Origin of Life considered in its bearing on 

 the Distribution and Migration of Birds. — Part II. By 

 H. B. Tristram, D.D., F.R.S. 



[Continued from 'The Ibis/ 1887, p. 242.] 



In suggesting tlie theory that the Polar origin of life affords 

 a key to the various problems connected with the distribution 

 and migration of birds, I do not think it necessary to go 

 further than I have already done into the question of past 

 changes in the eccentricity o£ the earth's orbit, since no such 

 change is required to account for the existing conditions of 

 life ; and in this, as in every other unsolved problem, the 

 simplest explanation generally proves the best. Nor does it 

 seem to me that we need speculate on the question of various 

 recurring glacial epochs in order to explain the phenomena 

 we encounter. I do not deny the existence of such alter- 

 nating periods of heat and cold ; I merely mean that their 

 introduction as postulates is not required for our present 

 purpose. 



I may refer to the summary in my first paper (Ibis, 1887, 

 p. 236), for which I am indebted to Mr. Scribner and Col. 

 Feilden, of the propositions as to the origin of life at the 

 Poles — the first portions of our cooling globe capable of 

 maintaining life. It is, moreover, important to divest our 

 minds of the popular notion that identical or similar forms 

 of life in geologic periods were synchronous. There is 

 abundant evidence, for example, that the Miocene flora of 

 Germany, though in many, or perhaps all, its species iden- 



