DEBIVATION OF BEITISH COLEOPTERA. 17? 



freedom from competition, either directly or indirectly 

 through food plants, &c, in certain suitable areas first 

 fixed the lines of migration and settlement. As however, 

 the available land surface became more and more crowded, 

 fixity of tenure would become more and more the order of 

 things, 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 retrogression and due 

 to human interference with environment. 



We can thus understand that, given an original exotic 

 immigration of an assemblage of species from the east 

 (firstly), 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 themselves through 

 natural selection, and that (secondly) 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. This explains why some species arrived in 

 Ireland while others were too late, and so 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 at 

 all became impossible. 



The characteristics of this second or Central European 

 group, then, are that from a maximum in the east and 

 south-east it thins out both specifically and individually 

 westward and northward, and if we find a species, as we 

 do several, whose maximum seems to be in Kerry, Corn- 

 wall, and South Wales, but is absent or rare in Norfolk, 

 Essex, and Kent, then we may, I think, assign such a 

 species ipso facto to our third group. 



