OOLEOPTERA. 271 



seems probable then to our imperfect knowledge is that this migration 

 from the east and south-east was subsequent to the maximum intensity 

 of glacial cold, but whether it was made possible by climatic conditions 

 or by geographical — that is to say, whether it was the gradual rise in 

 temperature that allowed the westward and northward march over a con- 

 tinuous area, or whether, climatic conditions being favourable, it was due 

 to an elevation of land which gave access from the continent to what had 

 previously been an island or assemblage of islands, of this there 

 appears to be no evidence. Nor does this very greatly affect the validity 

 of the hypothesis ; it is certainly more difficult to explain the different 

 rates at which species appear to have travelled, or more accurately 

 increased their range, for the process was probably quite imperceptible 

 when in operation. No doubt the correspondence between a species 

 new to any area and the environment, supply the factors which 

 determine the rate of its progress. An instance of how dissimilar such 

 a rate can be is furnished by a comparison between tljo extraordinary 

 increase of the European rabbit in Australia, and the apparently 

 insurmountable difficulties which attend the introduction of our 

 common English hare into Ireland, whose native form is a distinct but 

 closely allied species — the arctic or mountain hare — we may, I think, 

 safely assume that no species except under exceptional and non-natural 

 conditions, such as human interference or rearrangement of vegetation, 

 is now increasing, or perhaps ever will increase its range, at any rate 

 in Europe. No douljt fi-eedom from competition either directly or 

 indirectly, though food-plants in certain suitable areas, first ffxed 

 the lines of migration and settlement. As, however, the avail- 

 able land surface became moi-e and more crowded the rate of progress 

 would become slower, so that it seems probable that there must have 

 been a point somewhere in the past which marked the final equilibrium 

 of the forces of dispersal and the stress of competition, and that since 

 this point was reached, most movement has been in the form of retro- 

 gression and due to human interference with environment. 



We can thus understand that, given an original exotic immigration 

 of an assemblage of species into any previously unoccupied area, the 

 rate of progress initially perhaps high, would be continually decreasing 

 till it came to a dead stop, and that no length of time elapsing since 

 then would renew it, except by organic change in the species them- 

 selves through natural selection, and further, that the rate of advance 

 among the species themselves would be very unequal. In nature the 

 race is always to the swift, the battle invariably to the strong, and it 

 is thus possible to explain why some of the species we are considering 

 arrived as far west as ]\Iayo while others never got to Cheshire, or 

 arrived so late that they were cut off', or what is more probable, since 

 there are similar deficiencies in the Scotch fauna, that their rate of 

 progress was so slow that before they had got half this distance 

 progress of any kind became impossible. 



(TohcroiK-luilcd.] 



dfOLEOPTERA. 



The Variation and Distribution of the Genus Aphodius, 

 Illiger. By Frank Bouskell, F.E.H., F.R.H.S. [Read before 

 Section F. of the Leicester Literary and Philosophical Society] . — 



