426 NATURAL SCIENCE. dec, 



however, to take the book at the estimate provided, and by the 

 addition of a few compHmentary phrases to expand an advertisement 

 into an apparent appreciation. In Mr. Dixon's eyes the great point 

 of his book is that it sets forth a "new law of dispersaL" To be 

 brief, this new law is that species never extend their range south- 

 wards. It were unnecessary to enter upon a little disquisition as to 

 what a law means, and as to the folly of supposing that a generalisa- 

 tion from facts is any explanation of the faces. Mr. Dixon, we fear, 

 would not understand us ; and we should be performing a task 

 unnecessary in the case of most of our readers. A single quotation, 

 from many hundreds that might be made, will show that Mr. Dixon 

 is really making this elementary mistake. He writes (p. 57) " we 

 need no ' persistence through long epochs of barriers, isolating the 

 greater part of Africa from the rest of the world,' as Dr. Wallace 

 insists, to account for the absence of such groups as bears, moles, 

 camels, deer, goats, sheep, or such genera as Bos and Siis : a law 

 forbidding the southern emigration of such types is sufficient to 

 explain the facts, without invoking more or less hypothetical geo- 

 graphical obstructions." Cannot Mr. Dixon see, even if he had 

 succeeded in showing completely that southern dispersal never does, 

 or never has, taken place, that we should have found out no more 

 than a very interesting circumstance to be explained ? Dismissing, 

 however, as unworthy of notice what Mr. Dixon considers the point 

 of his book, we shall find in it a very judicious selection of facts and 

 arguments in support of the view that the dispersal of species has 

 taken place in a northerly direction. In the earlier part of his book 

 he concedes so much to common-sense as to endeavour to find a 

 rational basis for this absence of southerly dispersal. He holds that 

 species do not retreat from unfavourable conditions, at least in the 

 ordinary sense of the word. When the Glacial period came on, the 

 inhabitants of the ice-covered regions were simply exterminated. 

 Only those species survived which had a range extending far enough 

 south to avoid the ice. The rest of the species were, so to say, wiped 

 off the slate. He thinks that there were three great " refuge-areas," 

 or "range-bases," for inhabitants of North Europe during the periods 

 of glaciation. The northerly limit of one was the extreme southern 

 edge of the ice, and it extended across the English Channel, then 

 dry, and along France to the ice of the Alps and the ice of North 

 Spain. It was a land similar to the inhabitable Arctic regions of 

 to-day. The second refuge-area extended from south of the first to 

 the Sahara Sea. In this region, probably, most of the birds now in 

 England were preserved. The third area was to the south and east 

 of the second, extending down the Soudan to the Cape. In it were 

 preserved a smaller number of contemporary English species — those 

 chiefly with very wide and eccentric ranges of migration to-day. All 

 birds, thinks Mr. Dixon, which bred north of these refuge-areas, 

 were exterminated by the glaciation. All the birds of existing North 

 Europe have appeared there by a post-glacial, northern extension of 

 their breeding-grounds. 



Here is the whole subject-matter of Mr. Dixon's book ; and by a 

 vast array of facts and discussions of existing distributions and 

 migrations he attempts to support it. The maps and the facts are 

 both useful, and, as every intelligent reader will see that the " law " 

 part of the book is pure nonsense, we may honestly commend the 

 rest to discriminating persons. But we must admit that even such 

 will find the continual irruptions of the law very distracting, and 

 rather adapted to make them throw the whole thing aside. 



