GEOLOGICAL PALAEONTOLOGY 75 



an equable and unbroken record. On the other hand, 

 organic evolution proceeds with relative uniformity. 

 Acceleration and retardation, progress and reversion, 

 tend to qualify that statement in detail, but do not 

 affect its general truth. Just as wider stratigraphical 

 knowledge has bridged many of the gulfs that used 

 to be attributed to cosmic catastrophes, so further dis- 

 coveries among fossils have linked together many 

 groups previously regarded as widely separate. While 

 "saltations," or, for that matter, supernatural "special 

 creations," cannot be considered disproved, occasions 

 where such phenomena can be invoked grow steadily 

 fewer as research continues ; apparent breaks in the 

 sequence of evolution are more probably due to ignorance 

 than to reality. It may be doubted whether the actual 

 rate of evolution, if such exists as a definite principle, 

 will ever be determined with sufficient precision to 

 allow of actual " dating " of geological time ; but the 

 direction of its progress, in the main current and 

 tributary streams, is demonstrable enough to indicate 

 relative chronology. 



Since a geological period is the sum of its component 

 hemerae, clear realization of the significance of the part 

 will engender accuracy in definition of the whole. The 

 term " hemera " represents an essentially biological con- 

 ception, but has stratigraphical applications. In the 

 course of evolution of species, each lineage is believed 

 to attain a more or less prominent " acme," when its 

 morphological qualities are relatively static, its dis- 

 tribution commonly wide, and its numerical strength 

 marked. The components of a fauna do not attain 

 their acmes synchronously, but the several lineages rise 

 and fall, some rapidly and some slowly, with some 

 measure of independence. Selection may be made of 

 forms which show great acmaic prominence, and reach 



