FOSSILS— THEIR XATURE AND INTERPRETATION. IO9 



w'c find a few classes among the several subkingdoms, produc- 

 ing parts which could be preserved as fossils, but there are 

 reasons why even these are not present in abundance except 

 for a very few orders; the rest may be represented by here 

 and there a specimen, but only rarely, and any conclusions 

 drawn from their study will be conjectural to the extreme. 

 In the study of the laws of organic history it becomes neces- 

 sary, therefore, to make judicious selection of those classes of 

 organisms whose records are sufficiently abundant and con- 

 tinuous to furnish the desired evidence. 



Summary. — To summarize : When we study fossils in their 

 simple physical aspect, as mathematical forms in the rocks, 

 we find them presenting an orderly arrangement of sequence, 

 one after the other, in strict chronological order. When 

 classified by their likeness to each other into groups to form 

 natural species and genera, and when separated from each 

 other by their points of difference to form separate families, 

 orders, and classes, we find that there is the closest relation- 

 ship existing between the form they assume and the periods 

 of time when they lived. Taking a single suborder of the 

 Ccelenterata (the stony corals, or Madreporaria, with 448 

 known genera), of which fossil remains are found all the way 

 along, from the earliest fossil-bearing rocks to the sea-shores 

 of our modern ocean, we find all the genera relatively short- 

 lived, rarely exceeding the period of two systems in length of 

 duration, and the genera most nearly allied to each other in 

 f(M'm are always found in the systems chronologically nearer 

 to each other; and uniting tlie similar genera into families, 

 the families presenting greater contrast are found farther sepa- 

 rated chronologically from each other than from the families 

 presenting less strong contrasts. When we carry our study 

 further and interpret these fossils as the remains of organisms, 

 and say that they represent living organisms, we come face to 

 face with the fundamental law of organisms, that is, the law 

 of change and variation. All organisms have a history. So 

 unchangeable are the physical j^roperties of matter, so inx-ari- 

 able are the laws of crystallization of minerals, and so con- 

 stant are the chemical properties of substances, that any 

 irregularity in any of them at once suggests the influence of 



