THE GEOLOGIC FUXCTIOXS OF LIFE. 



It is e^^dent that whenever any geographic change introckices a 

 barrier to migration, the faunas of the dissevered portions will, in all 

 probabihty, develop along different lines, and will diverge into pro- 

 vincial faunas. On the other hand, any geographic change that unites 

 areas and leads to intermigration, tends to a community of famia or to 

 cosmopolitanism. These tendencies have been markedly felt all through 

 the geologic ages, and constitute one of the most vital features of their 

 history. When continents are connected, their faunas intermingle and 

 the exchange gives rise to common forms. They tend to blend into one 

 great fauna except so far as the local differences develop those minor 

 assemblages previously discussed. When continents are separated, they 

 tend to develop peculiar faunas, as do islands, but on a larger scale. 

 This is very obvious in the case of the land life, but needs more special 

 statement for the oceans. 



The oceans constitute a single body of water with ample connections 

 and stirred by a system of constant circulation. Probably tliis has 

 been true for most of known geologic time. A single cosmopohtan 

 fauna of the largest t^pe might be expected. Tliis is in a measure 

 realized in the pelagic fauna of the open ocean, though this is somewhat 

 modified by the climatic zones. But the marine famias that are fossil- 

 ized in the known strata, and have most geologic interest, are, T^ithrare 

 exceptions, not those of the open ocean, but those of the shore zones 

 and of the shallow seas. Xow, although these shore belts and shallow 

 seas are broadly connected vdih the great ocean body, and are usually 

 regarded as a part of it, they are singularly separated from it, or rather 

 thej^ are singularly separated by it, so far as the life dependent on 

 shallow-water concUtions is concerned. To this fife, the deep sea is a 

 barrier not quite as effective as the land, but still a barrier. The key 

 to tliis important fact may be found in a consideration of the vertical 

 distribution of life. 



The great horizon of hfe is at or near the contact zone of the atmos- 

 phere vvith the hydrosphere and hthosphere. Life dechnes with increas- 

 ing altitude, partly because of the lowering temperature, and partly 

 because of the increasing tenuity of the atmosphere. The successive 

 changes of plant and animal life with the ascent of mountains and 

 plateaus is familiar. Life dechnes in descent into the sea chiefly from 

 lack of light, and secondarily from the lowering of temperature. Light 

 is essential to the formation of chlorophyl and, through it, of all other 



