FOSSILS— THEIR NATURE AND INTERPRETATION. 83 



(5) Each species, like the individual, has a certain shorter 

 or longer life-period, after which it perishes, never to reappear. 



(6) From these principles it arises that the approximate age 

 of a stratum may be determined by the degree of similarity 

 of its fossils to the forms of the present time. The fossils 

 contained in the strata are the means of determining the 

 equivalency (that is, likeness of age) of the strata themselves, 

 and in general, identical fossils indicate contemporaneity of 

 the enclosing strata. 



Change of the Forms of Fossils with Passage of Time, and 

 particular Form characteristic of Particular Periods of Time, 

 undeniable Facts of Paleontology. — Thus it appears that what- 

 ever we make out of fossils, whether we consider them stones 

 or organisms, however we account for their origin, whatever 

 relation we conceive them to bear to each other, the fact is 

 startlingly vivid to the paleontologist that the form of a fossil 

 is intimately associated with the time in which it appeared on 

 the earth ; that the morphological characters assumed by fos- 

 sils have been gradually and incessantly changing from the 

 beginning of the world. 



Inorganic Things, on the contrary, Unchangeable, — This is 

 contrary to the law in respect of every inorganic thing. The 

 chemical composition of things and the chemical properties 

 are the same so far back as we can trace them and to the 

 most distant star in space. Minerals in the Archaean ages, 

 before any fossils had appeared, crystallized out into exactly 

 the same forms which they assume to-day. We know of not 

 the least fluctuation in the laws of physics for all time. 

 Indeed, it is by dependence upon the absolute certainty and 

 uniformity of these laws that the astronomer is able to calcu- 

 late the position, the size, and the orbit of some unknown 

 and unseen planet, and directing his telescope to the place 

 where it should be, to discover it there. 



Fossils characteristic of Particular Periods of Geologic Time. 

 — The morphological combination of characters, which we call 

 a fossil (as a Trilobite or an Ichthyosaurus), has its definite 

 relationship to geological time, and each form is characteristic 

 of a particular period of time. A fossil becomes the unmistakable 

 mark of the age of the rock in which it is enclosed: the Trilo- 



