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also, for almost any number of years ; but if, from any of the innumerable causes 

 which aflfect the conditions of life, some change is made in the surroundings of 

 any class of plants or animals, that class of plants or animals is sure, before very 

 long, either to die out altogether, or to alter materially, so as to adjust themselves 

 to the new conditions. Now from these remarks it will easily be seen that, owing 

 to some regions of the earth having been much more disturbed and altered in their 

 physical character than others, the animals and plants which have lived upon the 

 disturbed regions will also show a much greater variety of type and form than 

 those which have lived upon a quiet and undisturbed region ; and hence, looking 

 back through the strata of rocks, a great succession and variety of forms in the 

 rocks of some countries, and a great uniformity of form in the rocks of other 

 countries, and the forms of animal life found in the upper strata of some regions 

 may be somewhat similar to those found in the lower strata of other regions, and 

 from these appearances it might be argued, and, indeed, frequently has been 

 so argued, that these two sets of rocks have been formed at very different periods 

 of the world's history, the truth being, that in one set of rocks frequent changes of 

 animal and plant species were going on, while in other rocks, undisturbed by 

 changes, the same species lived and died from century to century, and that as 

 a matter of fact, the two sets of rocks were being formed contemporaneously. It 

 must, therefore, be clear, from the foregoing remarks, that it is exceedingly 

 difficult to pronounce upon the age of any rocks upon the authority of their animal 

 or vegetable remains. It is not the object of this paper to go into details, but I 

 may just mention, by way of example of the foregoing remarks, that if we take a 

 belt of the earth's surface, from, say, latitude 55° N. and longitude 10° W., and 

 about 20° in width, to latitude 5° S , and longitude 150° E., we will pass over an 

 area of great commotion and disturbance, and we will find a corresponding amount 

 of change in the animal and vegetable inhabitants of these regions, and if we take 

 a broad belt south of the other, we will pass over a large tract of land where 

 nature seems to have been, for untold ages, going forward in a quiet and unmo- 

 lested routine, and here we find but slight changes in the animal and vegetable 

 remains. The first-mentioned belt would take in England and Wales, Europe, 

 south of the Alps and Balkans, and down to the shores of Africa, Asia Minor, 

 Persia, North of India and the Himalayas, Malay and the Malay Archipelago ; 

 and the second belt would embrace Central and North Africa, Southern India, 

 and Australia. I may just point out that all civilisation had its origin within the 

 first-named belt of country. From these few remarks upon the causes of change 

 in animal and plant life, it will be seen how very possible it is for whole genera- 

 tions of animal or vegetable forms to pass away and become extinct in one region 

 of the earth, while during the same period, in another, the same old forms continue 

 to live and flourish, and, therefore, we must be very cautious, indeed, as to how 

 far we lay down the law as to any measurement of time based upon the number 

 of species which may be somewhere found as fossils. 



