CHAP. XXIV SUMMARY AND CONCLUSION 539 



proportionate change of the species of moUusca affords 

 a means of arriving at the whole lapse of time represented 

 by the fossil iferous rocks ; and these two estimates agree 

 in the order of their magnitudes. 



It is then argued that the changes of climate every 

 10,500 years during the numerous periods of high 

 excentricity have acted as a motive power in hastening on 

 both geological and biological change. By raising and 

 lowering the snow-line in all mountain ranges it has 

 caused increased denudation ; while the same changes 

 have caused much migration and disturbance in the organic 

 world, and have thus tended to the more rapid modification 

 of species. The present epoch being a period of very low 

 excentricity, the earth is in a phase of exceptional stability 

 both physical and organic ; and it is from this period of 

 exceptional stability that our notions of the very slow rate 

 of change have been derived. 



The conclusion is, on the whole, that the periods allowed 

 by physicists are not only far in excess of such as are 

 required for geological and organic change, but that they 

 allow ample margin for a lapse of time anterior to the 

 deposit of the earliest fossiliferous rocks several times 

 longer than the time which has elapsed since their deposit 

 to the present day. 



Having thus laid the foundation for a scientific interpre- 

 tation of the phenomena of distribution, we proceed to the 

 Second Part of our work — the discussion of a series of 

 typical Insular Faunas and Floras with a view to explain 

 the interesting phenomena they present. Taking first two 

 North Atlantic groups — the Azores and Bermuda — it is 

 shown how important an agent in the dispersal of most 

 animals and plants is a stormy atmosphere. Although 900 

 and 700 miles respectively from the nearest continents, 

 their productions are very largely identical with those of 

 Europe and America ; and, what is more important, fresh 

 arrivals of birds, insects, and plants, are now taking place 

 almost annually. These islands afford, therefore, test 

 examples of the great dispersive powers of certain groups 

 of organisms, and thus serve as a basis on which to found 

 our explanations of many anomalies of distribution. Passing 



