CONTEMPORANEITY AND HOMOTAXIS. 49 



from one or other of these areas, or from some point still unknown ; 

 and this dispersal must necessarily have consumed a long period of 

 time. Any other view would lead us almost inevitably to the now 

 abandoned theory that each period in geological history was char- 

 acterised by a special group of organisms spreading over the whole 

 globe, and that there took place at the close of each period a general 

 destruction of all existing forms of life, and a fresh creation of the 

 new forms characteristic of the next period. 



In our inability, then, to accept this view, we must seek for some 

 other explanation of the observed facts. The most probable view, 

 and the one which is supported most strongly both by what we see 

 at the present day and by what we learn from numerous examples in 

 past time, is this : The Carboniferous Limestone — to take this mem- 

 ber of the Lower Carboniferous deposits in particular — was not 

 deposited all over the world in one given period, by one sea, or at 

 exactly the same time ; so that it cannot be said to be strictly " con- 

 temporaneous " wherever it is found. This would imply a uniformity 

 of conditions over vast distances, such as exists nowhere at the present 

 day, and such as we have no right to assume ever existed. On the 

 contrary, the deposition of the Carboniferous Limestone must have 

 first taken place in one comparatively limited area — say in Europe — 

 where fitting conditions were present both for the animals which 

 characterise it, and for the formation of beds of its peculiar mineral 

 and physical characters. How wide this area may have been, signifies 

 very little. It may have been as large as the area now covered by 

 the Pacific, or larger, and yet it could not include all those localities 

 in which strata of Lower Carboniferous age with identical or repre- 

 sentative fossils are already known to exist. Under any circum- 

 stances, some dispersion of the species of the original Carboniferous 

 area must have been going on by the ordinary processes of migration 

 from the commencement of the Carboniferous period, but this dis- 

 persion must have been greatly accelerated towards the close of the 

 period of the deposition of the Carboniferous Limestone. At this 

 time the conditions present in the original area must be supposed to 

 have become unsuitable for the further existence in that area of the 

 assemblage of animals which had been its inhabitants, or, at any 

 rate, for a great many of them. The change from suitable to unsuit- 

 able conditions must, it is hardly necessary to say, have been an 

 extremely slow and gradual one, and would doubtless be connected 

 with the progressive shallowing of the sea, the diversion of old cur- 

 rents of heated water, or the incoming of new currents of cold water, 

 or other physical changes tending to alter the climatic conditions of 

 the area. What, then, would be the effect of such a change of con- 

 ditions as we have supposed, upon the animals inhabiting the area ?' 

 — Some of them would, doubtless, be sufficiently hardy and accom- 



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