184 PRINCIPLES OF CLASSIFICATION [Ch. XIII. 



sued by the naturalist in the construction of genera, when he selects a 

 typical species, and then classes as congeners all other species of animals 

 and plants which agree with this standard within certain limits. The 

 genera A and C having been founded on these principles, a new species 

 is afterwards met with, departing widely both from A and C, but in 

 many respects Of an intermediate character. For this new type it be- 

 comes necessary to institute the new genus B, in which are included all 

 species afterwards brought to light, which agree more nearly with B than 

 with the types of A or C. In like manner a new formation is met with 

 in geology, . and the characters of its fossil fauna and flora investigated. 

 From that moment it is considered as a record of a certain period of the 

 earth's history, and a standard to which other deposits may be com- 

 pared. If any are found containing the same or nearly the same organic 

 remains, and occupying the same relative position, they are regarded in 

 the light of contemporary annals. All such monuments are said to re- 

 late to one period, during which certain events occurred, such as the 

 formation of particular rocks by aqueous or volcanic agency, or the con- 

 tinued existence and fossilization of certain tribes of animals and plants. 

 When several of these periods have had their true places assigned to 

 them in a chronological series, others are discovered which it becomes 

 necessary to intercalate between those first known ; and the difficulty of 

 assigning clear lines of separation must unavoidably increase in propor- 

 tion as chasms in the past history of the globe are filled up. 



Every zoologist and botanist is aware that it is a comparatively easy 

 task to establish genera in departments which have been enriched with 

 only a small number of species, and where there is as yet no tendency 

 in one set of characters to pass almost insensibly, by a multitude of con- 

 necting links, into another. They also know that the difficulty of classi- 

 fication augments, and that the artificial nature of their divisions becomes 

 more apparent, in proportion to the increased number of objects brought 

 to light. But in separating families and genera, they have no other al- 

 ternative than to avail themselves of such breaks as still remain, or of 

 every hiatus in the chain of animated beings which is not yet filled up. 

 So in geology, we may be eventually compelled to resort to sections of 

 time as arbitrary, and as purely conventional, as those which divide the 

 history of human events into centuries. But in the present state of our 

 knowledge, it is more convenient to use the interruptions which still 

 occur in the regular sequence of geological monuments, as boundary 

 lines between our principal groups or periods, even though the groups 

 thus established are of very unequal value. 



The isolated position of distinct tertiary deposits in different parts of 

 Europe has been already alluded to. In addition to the difficulty pre- 

 sented by this want of continuity when we endeavor to settle the chrono- 

 logical relations of these deposits, another arises from the frequent 

 dissimilarity in mineral character of strata of contemporaneous date, 

 such, for example, as those of London and Paris before mentioned. The 

 identity or non-identity of species is also a criterion which often fails us. 



