244 TOWARDS THE HUMAN FORM 



oscillatory movements of land surface levels were accentuated 

 to such a degree during the Cretaceous Period that a deep 

 fold, invaded by the sea, was produced right across Europe, 

 then consisting of a series of archipelagoes, from east to west, 

 and across the North American continent from north to south, 

 cutting it in two, and also across Africa, whose western portion, 

 now projecting into the Atlantic Ocean, was thus cut off from 

 the rest of the continent. It was then that the shifting and 

 derangement of strata took place known as the dislocation of 

 the chalk. Elsewhere, however, the sea retreated, leaving 

 behind it lagoons which dried up and left evidence of their 

 presence in deposits of salt, while the summits emerging from 

 the water formed islands and temporary archipelagoes. New 

 communication was thus established between the different seas 

 and former communication was cut off. The faunas hitherto 

 separated became mixed in some districts and in others 

 was isolated in groups and thus forced to follow individual 

 lines of evolution. Hence a greater variety of marine species 

 resulted, and many southern species were carried north by 

 currents passing through the new straits, whereas northern 

 species penetrated to the south, so that we cannot arbitrarily 

 assume persistent variations of the mean temperature from 

 the presence of such species in given waters. Such changes, 

 however, were of minor importance and in no way disturbed 

 the universal calm. 



In every respect the transition from the Primary to the 

 dawn of the new era was effected gradually. Throughout the 

 Triassic Period the vegetation did not differ, except in details 

 of genera and species, from that of the Primary ; nor would 

 it seem at a first glance, at least, judging by our information 

 at present, to have been greatly modified during the subsequent 

 Jurassic Period. It is probable, however, that it was during 

 this era that the fertile female leaves, which remained open 

 in the Gymnosperms, in certain instances coiled up and closed 

 around their ovules in order to protect them : a decisive step 

 forward was thus taken in the Vegetable Kingdom — Angio- 

 sperms had been evolved. At all events, they were abundant 

 and varied from the beginning of the Cretaceous, and the 

 families that made their appearance first were precisely those that 

 we should expect to do so according to the theory enunciated 

 on p. 106. Dicotyledons were greatly in the majority, and 



