CHAP. I.] INTRODUCTORY. 



slow process of development or trausniutation, all animals have 

 been produced from those wliioh preceded them ; and the old 

 notion that every species was specially created as they now 

 exist, at a particular time and in a particular spot, is abandoned 

 as opposed to many striking facts, and Unsupported by any 

 evidence. This modification of animal forms took place very 

 slowly, so that the historical period of three or four thousand 

 years has hardly produced any perceptible change in a single 

 species. Even the time since the last glacial epoch, which on 

 the verj)' lowest estimate must be from 50,000 to 100,000 years, 

 has only served to modify a few of the higher animals into very 

 slightly different species. The changes of the forms of o,nimals 

 appear to have accompanied, and perhaps to have depended 

 on, changes of physical geography, of climate, or of vegetation ; 

 since it is evident that an animal which is well adapted to one 

 condition of things will require to be slightly changed in con- 

 stitution or habits, and therefore generally in form, structure, or 

 colour, in order to be equally well adapted to a changed 

 condition of surrounding circumstances. Animals multiply so 

 rapidly, that we may consider them as continually trying to 

 extend their range ; and thus any new laud raised above the 

 sea by geological causes becomes immediately peopled by a 

 crowd of competing inhabitants, the strongest and best adapted 

 of which alone succeed in maintaining their position. 



If we keep in view these facts — that the minor featuies of the 

 earth's surface are everywhere slowly changing; that the forms, 

 and structure, and habits of all living things are also slowly 

 changing ; while the great features of tlie earth, the continents, 

 and oceans, and loftiest mountain ranges, only change after very 

 long intervals and with extreme slowness; we must see that 

 the present distribution of animals upon the several parts of 

 the earth's surface is the final product of all these wonderful 

 revolutions in organic and inorganic nature. The greatest and 

 most radical differences in the productions of any part of the 

 globe nmst be dependent on isolation by the most effectual 

 and most permanent barriers. That ocean which has remained 

 broadest and deepest from the most remote geological epoch 



