48 PRINCIPLES OF PALEONTOLOGY. 



capable of subdivision into a number of definite rock-groups or 

 " formations, " each possessing a peculiar and characteristic 

 assemblage of fossils, representing the "life" of the "period" in 

 which the formation was deposited. We have still to inquire 

 shortly how it came to pass that two successive formations 

 should thus be broadly distinguished by their life-forms, and 

 why they should not rather possess at any rate a majority of 

 identical fossils. It was originally supposed that this could be 

 explained by the hypothesis that the close of each formation 

 was accompanied by a general destruction of all the living 

 beings of the period, and that the commencement of each new 

 formation was signalized by the creation of a number of brand- 

 new organisms, destined to figure as the characteristic fossils of 

 the same. This theory, however, ignores the fact that 

 each formation as to which we have any sufficient evidence 

 contains a few, at least, of the life-forms which existed in the 

 preceding period; and it invokes forces and processes of which 

 we know nothing, and for the supposed action of which we can- 

 not account. The problem is an undeniably difficult one, and it 

 will not be possible here to give more than a mere outline of 

 the modern views upon the subject. Without entering into the at 

 present inscrutable question as to the manner in which new life- 

 forms are introduced upon the earth, it may be stated that almost 

 all modern geologists hold that the living beings of any given 

 formation are in the main modified forms of others which have 

 preceded them. It is not believed that any general or universal 

 destruction of life took place at the termination of each geolog- 

 ical period, or that a general introduction of new forms took 

 place at the commencement of a new period. It is, on the con- 

 trary, believed that the animals and plants of any given period 

 are for the most part (or exclusively) the lineal but modified 

 descendants of the animals and plants of the immediately pre- 

 ceding period, and that some of them, at any rate, are continued 

 into the next succeeding period, either unchanged, or so far 

 altered as to appear as new species. To discuss these views in 

 detail would lead us altogether too far, but there is one very 

 obvious consideration which may advantageously receive some 

 attention. It is obvious, namely, that the great discordance which 

 is found to subsist between the animal life of any given forma- 

 tion and that of the next succeeding formation, and which no 

 one denies, would be a fatal blow to the views just alluded to, 

 unless admitting of some satisfactory explanation. Nor is this 

 discordance one purely of life-forms, for there is often a physical 



