386 



PRINCIPLES AND METHODS OF PALAEONTOLOGY. 



from those wliicli now obtain; and we mnst, therefore, be content to 

 regard many of the established generalizations as only approximatively 

 correct. 



As a general rule, however, it is very true that the more we learn of 

 the world of fossils, the more clearly does the conviction force itself 

 upon our minds, that from the earliest times of which we possess a rec- 

 ord to the present, no change has taken i^lace in the general scheme of 

 the organic world. There are perhaps 15,000 established species of 

 extinct animals, but among them there is not one whose plan of con- 

 struction difl'ers so far from any now known, as to require the estab- 

 lishment of even a new class for its reception. Diiferent naturalists will 

 estimate the number of classes of animals now living variously ; but 

 they may be safely assumed to be at least tive-and-twenty distinct mod- 

 ilications of the live great primitive common plans ; and yet so com- 

 paratively slight has been the change since the earliest times, that the 

 NA'hole extinct world will not supply us with a six-aud-twentieth modifi- 

 cation. If we descend to the next smaller divisions, to the orders, the 

 same fact becomes apparent ; at the very lowest estimate there are 

 not fewer than between a hundred and thirty and a hundred and forty 

 orders of animals, and out of these, at the most, not more than fourteen 

 or fifteen are represented only by extinct forms ; that is to say, in the 

 whole range of geological series not more than ten or twelve per cent, 

 of ordinal tyi)es, different from those which now exist, having come into 

 being.* 



4. The history told by the records of the organic world is in perfect 

 harmony with that which is written on the face of inorganic nature. 

 Tlie thickness of the crust of the earth, down to the greatest depths to 

 which man has been enabled to penetrate, is to a great extent composed 

 of strata of rock, the physical and chemical peculiarities of which 

 evince their identity with the products of the present oi)erations of 

 nature. Beds of conglomerate containing round pebbles demonstrate 

 that the sea beat against and broke up its rocky boundaries then as 

 now, rounding and polishing the fragments by incessant friction as it 

 wears them on any modern shingle beach ; fine-grained limestones and 

 sandstones show that, then as now, the finer products of their attrition 

 were carried away and deposited, in the form of beds of mud, upon the 

 deeper and (]uieter parts of the sea bottom. Vast and frequent inter- 

 ruptions in the regular series of l)ed prove that, in ancient times as at 

 present, tlie solid crust oscillated, so that what was dry land became 

 covered by the sea, and what was sea bottom remained for long ages 

 dry land. And, finally, in like manner as we know that, within the 

 period of which man is cognizant, all these changes have gone on in an 

 excessively slow and gradual manner, rapid and convulsive action being 

 altogether exceptional, so we have the clearest proof that the time repre- 

 sented by the vast succession of ancient strata is enormous and almost 

 inconceivable, and that gradual and regular change was, then as now, 

 the rule, catastrophe and convulsion the exception. Nevertheless, as 

 in the ancient organic world we have found that there is a certain amount 

 of departure from what might be called the by-laws of the present cre- 

 ation, so it is quite possible that, in the physical world of past times, 

 changes may have now and then taken place with a rapidity and a 

 violence to which the minute experience of man affords no parallel. An 

 Ichthi/osaurus is, in one sense, a sort of animal catastrophe, and as we 

 are all well assured of the occurrence of this one wide deviation from 



* The number of orders is liero purposely taken at an extreme minimum ; while the 

 highest possible value is given to the extinct groups. 



