HA RD WICKE' S S CIENCE- G OS SIP. 



25 



NOTES ON GEOGRAPHICAL DISTRIBUTION. 



By T. D. A. COCKERELL. 



S year by year 

 Science becomes 

 more complicated, 

 and the array of 

 ascertained facts so 

 vast as to be far 

 beyond the grasp 

 of any single brain, 

 specialism must of 

 necessity exist, and 

 not only this, but 

 to know anything 

 thoroughly one 

 must be more cr 

 less a specialist. 



Nor is this to 

 be regretted, since 

 nearly all the 

 knowledge of later 

 days has been 

 gained from work- 

 ers in particular groups, and it is certainly true, 

 that the man who knows one group well knows the 

 principles of life, heredity, variation and distribution, 

 so far as they can be known, and is able to apply 

 these principles with nearly equal facility to other 

 groups. Yet there is one kind of specialism very rife 

 that I do protest against, as being productive of 

 inconceivable misconception and narrowness of mind ; 

 that specialism which studies a group only as it occurs 

 in a particular country or district, and cares nothing 

 for its distribution or variation without that limit. 

 And in thus protesting, I do not wish to say anything 

 against local lists, which to me have the greatest 

 value, or against those who give most attention to 

 the fauna of their own district, for it is most natural 

 and right that each should study what comes most 

 directly under his observation, but against the feeling 

 of contempt for all foreign species, and that ignorance 

 which knows that Vanessa antiopa is a very rare 

 butterfly which used formerly to be found at Camber- 

 well, and thinks there is nothing more to know. 

 No. 278.— February 1888. 



My object, therefore, in this paper, is to rouse up 

 some measure of interest in the foreign distribution of 

 our British species among those who have hitherto 

 regarded them from the British standpoint alone, and 

 to show how we owe much of our native fauna to 

 immigration, and on the other hand have from time 

 to time sent forth emigrants to people other lands. 



In a former note I attempted to divide up the 

 British Mollusca into three sections, the Northern, 

 Eastern, and Western ; or otherwise, the Boreal, 

 Germanic, and Lusitanic, and asked for adverse 

 criticisms upon this arrangement, so that it might be 

 put to the test. Dr. Kobelt, in an article published 

 in Germany, took exception to the classification of a 

 few species, but agreed with it in the main, and 

 beyond this nobody seems to have disputed my 

 division of our fauna, which is based upon that 

 proposed by Forbes many years ago. 



The circumpolar region it maybe supposed, was at 

 one time temperate in climate, and possessed a fairly 

 uniform fauna and flora. Cold coming on, drove all 

 forms of life southward, and hence the same species 

 appeared simultaneously in Central Europe and Central 

 North America, meeting and mingling with the" 

 fauna of the south — and thus was formed our British 

 boreal fauna and flora. 



Next, the question arises, how are the boreal types 

 to be distinguished from the eastern and western ? 

 Their distribution in Britain is certainly some guide 

 to this ; the northern forms being mainly prevalent in 

 Scotland and the North and West of Ireland, in which 

 last district they mingle with the western ; but 

 Planorbis parvus — certainly a boreal type, is found in 

 Devonshire and Surrey, and Cochlicopa lubrica is 

 universal. One test alone seems to me a sure one, 

 and that is by comparison with the North American 

 fauna : — Whatever form in Britain is represented in 

 North America by an identical or very nearly allied 

 species belongs to the boreal type, except such as have been 

 introduced by human agency, and perhaps certain 

 migratory species. 



Possibly, it does not always exclude a species from 



c 



