RELATION OF BIOLOGY TO GEOLOGICAL INVESTIGATION. 299 



It can not be reasonably doubted thai ;is a rule the specific forms 

 which constitute the fossil aquatic fauna of any stage <>v substage of 

 the geological scale reached the localities where they are found by 

 geographical dispersion from a single genetic center, even in cases of 

 unusually great dispersion. Slill.it seems impossible that all the fossil 

 forms which geologists usually feel obliged to regard as representing 

 separate species could have originated and become dispersed in that 

 manner. It therefore may be reasonably assumed that each of the 

 really or apparently identical forms which occur in different regions, 

 but which belong to one certain stage or substage, may have reached 

 their respective geographical stations within that stage or substage by 

 separate evolutional lines from a common ancestral form which existed 

 in a preceding stage, which lines were too slightly differentiated to 

 produce new specific characteristics. In short, paleontological evidence 

 seems to warrant the conclusion that in many cases, at least, both 

 generic and specific forms have originated independently in different 

 parts of the world, not only contemporaneously, but at successive inter- 

 vals of time. 



If species and genera really had such a diversity of origin as has 

 been suggested, the various types which they constitute and which are 

 held to characterize the various stages ami sitbstages of the geological 

 scale maybe assumed to have originated in a similarly diverse manner. 

 Furthermore, the variable rate of differential evolution suggests a rea- 

 son why certain of the characteristic types of a given stage or substage 

 might naturally have survived the others and continued their existence 

 into the next substage, as indicated in the paragraph following propo- 

 sition 0. 



(9) The animal and vegetable life of cadi stage of the geological scale was in the 

 aggregate different as to its forms from that of all others, ai.d each stage and sub- 

 stage was further specially characterized by certain generic, and also more general, 

 types or peculiar groups of species. These types, however, were not necessarily 

 confined within absolute time-limits. 



So distinctive are the assemblages of types of organic forms which 

 characterize each of the stages or systems of stratified rocks that, not- 

 withstanding the exceptions mentioned in preceding paragraphs, the 

 experienced geologist upon such evidence alone readily assigns to its 

 proper stage of the great geological scale comprehensive collections of 

 fossil remains from any given series of stratified rocks in any part of 

 the world. For example, the great Carboniferous system has been by 

 means of its fossils as distinctly recognized in Asia and in North and 

 South America as in Europe where it was first studied, and in all those 

 parts of the world it has been supposed to be sharply definable and 

 wholly distinct, as to its fossil forms, from the Triassic above and the 

 Devonian beneath. Later investigations, however, have shown that 

 Devonian and Carboniferous types are often commingled upon the 

 lower, and Carboniferous and Triassic types upon the upper, confines 

 of the Carboniferous system. 



