8o PRINCIPLES OF STRATIGRAPHY 



2,S'44-) (See, further, under Chapter XXIX.) Although the 

 distribution of the genera of ammonites, as well as of corals, does 

 not follow so rigidly the belt-like arrangement in extra-European 

 countries as held by Neumayr, and although much of the distribu- 

 tion is explainable by difference in depth, still the probability of 

 differentiation of the earth's surface into climatic zones is not to be 

 altogether discarded. A part of the anomalous distribution of 

 ammonites may certainly be explained by floatation (see Chapter 

 XXVni), while the deflection of the zonal boundaries, due to conti- 

 nental expansions or contractions, must also be considered. Finally, 

 the influence of ocean currents in modifying the temperature of the 

 water must not be neglected. Ortmann {yi:26y) especially em- 

 phasizes the occurrence of reef-building corals in the Russian Jura, 

 in a region included in Neumayr's boreal zone, and insists that 

 these indicate a tropical climate, but Neumayr contended that such 

 occurrence was not sufficient evidence against his theory. 



That climatic zones existed during Mesozoic times, if not ear- 

 lier, is not at all an improbable supposition, though it may perhaps 

 be questioned whether the differences between successive zones 

 were always as great as now. That the air temperature as a whole 

 was higher at times than now, probably through an increase in the 

 carbon dioxide content and the consequent trapping of solar heat, 

 is highly probable, and indeed seems far more reasonable than the 

 assumption of a uniform climate over the entire earth. A universal 

 rise in temperature of the air would permit a wider distribution of 

 those marine types now restricted to the tropical water of to-day, 

 for the present tropical temperatures would extend to higher lati- 

 tudes. Even though the tropics under such conditions would be 

 much more highly heated, it is questionable if the waters would be 

 too hot for the existence of life. It would only be necessary for 

 organisms living in the tropics to descend to greater depths, so as 

 to escape the excessive surface temperatures. Nor must the pos- 

 sibility be overlooked that stenothermal modern forms may have 

 had eurythermal ancestors; that, in other words, the descendants 

 of once widely adapted classes of organisms, capable of existing 

 under a great range of 'temperature, have now become restricted to 

 a limited range, in the warmer waters of to-day. 



It is a universally recognized fact that climatic conditions of 

 greater severity existed in the recent geologic past, so that exten- 

 sive portions of the northern continents became glaciated. The 

 very fact that these glaciers extended over only part of the earth's 

 surface shows the existence of climatic zones, the limit of glacia- 

 tion marking the poleward limit of a milder belt. The southern 



