136 f-^""^' 



explain that gap would be a submergence of the whole of our central 

 area — a submergence for which, in spite of the existence of marine 

 shell beds* of Pleistocene age on the Welsh Mountains, there is no 

 warrant subsequent to the separation of England from Ireland. In 

 any case, a submergence of the extent recjuired would have cleared 

 the insect out of the southern English localities far more effectively 

 than from the higher birch-clad regions of the North of England, 

 where both birch, and particularly alder, are very common indeed 

 In that direction there is no explanation for the gap, if its distri- 

 bution has once been continuous. We must therefore search for 

 other reasons. Human agency might be considered, but the only 

 causes which one can suggest are intensity of cultivation and the 

 firing of primaeval forests, both of which causes acted with far 

 greater severity in the South of England than in the JSTorth, and, on 

 that supposition, the only places in England to contain the insect 

 would be the very ones from which it is so evidently absent. 



However, all these surmises are based on the usual idea that 

 our islands are colonised chiefly by plants and animals advancing 

 from the south, whether south-westerly or south-easterly — an idea 

 not supported by the present facts of distribution. - Excluding those 

 cases of erratic or unexpected distribvition exhibited by the American 

 and Lusitanean elements of our flora and fauna, which, to any but 

 the perfervid glacialist, are palpably survivals from our Pliocene 

 flora and fauna, and therefore cannot be brought under any rule, 

 the ranges of the bulk of our plants and animals are ordered and 

 regular, suggesting that our Northern forms came from the North 

 and the Southern forms from the South, and so on. Or, more plainly, 

 the range of our Northern animals, etc., is Northern and Western, 

 whilst that of our Southern ones is just as certainly Southern and 

 Eastern. One need only point to the stations of such things as the Pearl 

 Mussel (JJnio margaritifer h.), the Carabid beetle Pelophila horealis 

 Payk., the water-beetle Dytiscus lapfonicus G-yll., the Coccid Orthezia 

 cataphracta Shaw, and the large Heath Butterfly (Coenonympha tiphon 

 Rett.), as proofs of the first statement, and to the cases of the 

 whole group of the Wave Moths (Acidaliae), the Painters' Mussel 

 (JJnio pictoriim L.), the Spurge Laurel {Daphne latireola L.), to 

 confirm the second. These are but a few examples chosen at ran- 

 dom from the enormous number available in all groups to illustrate 

 the point. 



* See Wright, "Ice Age in North America," page 401. 



