96 [April, 1917. 



entirely different routes, the two gi'oups of the immigrant species 

 never linking up. In this way we have in Britain, according to 

 Mr. J. W. H. Harrison,* who is an extremely able student of Zooge- 

 ography, two distinct and separate immigrant groups of the Kentish 

 Glory Moth Dimorpha {Endrnmis) versicolora L., one in the North of 

 Scotland, which made its way from Siberia by way of the Scandi- 

 navian peninsula and its past connection with our islands, and the 

 other in the south of England, which has come by way of the 

 Continent. 



8. — Exceptional Means of Distribution. 



Finally, in order to explain certain other examples of discontinuous 

 distribution, the aid of special extraneous causes has often been invoked, 

 such as mud on the feet, or food in the crops of migratory birds, 

 ocean currents, floods, gales, etc. Unfortunately, though these 

 agencies do work to some extent, and in certain cases, there is a great 

 conflict of opinion on the subject, largely owing to the lack of proof 

 that some of the methods are even possible. It is said, for example, 

 that birds migrate on empty stomachs, and therefore could not carry 

 seeds or resistent eggs in their crops, and that immersion in sea- 

 water is fatal to many animal forms. Under the circumstances, then, 

 it will be unnecessary here to do more than merely mention the 

 suggestion. 



B. — Ontogenetic Factors. 



We now come to a series of phenomena which are familiar to 

 every student of nature, namely, that species do not occur in equal 

 numbers every season ; that in some years what are usually quite 

 common species are almost rare, and at other times so-called rarities 

 are quite common. Almost every species shows a kind of irregular 

 periodicity of this kind,t which must evidently be a result of circum- 

 stances which have occurred during the life of at most a few genera- 

 tions. In all probability, the fundamental determining factor in these 

 cases is always one of climate, reacting either directly on the organism 

 in one of its stages, or indirectly through its enemies, whether com- 

 petitive, pi'edatory, or parasitic. 



* "The Geogra))hical Distribution of Dimorpha (Bntlromin) vtrsicolor." Ent. Mo. Mag., 1916, 

 pp. 131-139. 



t Cr'. W. E. Shai]!. " Hiidi-adiphaga of Lanca-sliire and Cheshire." Naturalist, 1892, pp. 

 189-190. 



