384 Mr. S. V. Wood on the Effects upon Animal Life 



trade-winds into monsoons, which at the present day invariably 

 occur in those continents that arc washed on their southern 

 border by tropical seas, would be no fit abode for terrestrial 

 animals brought into existence under the opposite class of con- 

 ditions, as were those which came into being during the long 

 secondary period ; and, as I shall attempt presently to show, the 

 effect of the post-cretaceous changes was to raise into greater 

 importance, during the tertiary period, those forms of terrestrial 

 or fluviatile animals that are by their habits suited to sustain 

 these altered conditions, and to destroy those which could sup- 

 port life only under the conditions of humidity and equable tem- 

 perature prevailing when they came into existence. 



Passing, however, for the present to the changes presented in 

 marine animal life, we find one remarkable feature at the dawn 

 of the tertiary period which appears to me to afford a clue to the 

 entire change in marine vertebrate life that took place during 

 the intra-cretaceous and tertiary interval ; I allude to the disap- 

 pearance of the tetrabranchiate family of Cephalopoda, with the 

 sole exception of the Nautilus and Aturia, and to the preserva- 

 tion, and perhaps increase, of the dibranchiate Cephalopoda. 

 We know that the Nautilus is a bottom feeder, and therefore esc 

 necessitate a shore-follower ; and there is reason to infer that the 

 AmmonitidaB and other chambered Cephalopods of the newer 

 secondary formations had similar habits : the abundance of these 

 forms in those secondary formations which, like the lias, oolite, 

 and cretaceous formations of England and Northern France, were 

 deposited under littoral conditions, and in a partially land-locked 

 gulf, supports this inference. We cannot suppose these cham- 

 bered Cephalopods to have had habits in any way resembling the 

 Dibranchiata, which at the present day are surpassed by no animal 

 in their distribution over the ocean. This disappearance of the 

 AmmonitidaB and preservation of the Nautilidse, we may infer 

 was due to the entire change which took place in the condition 

 of the shores at the close of the cretaceous period; and this 

 change was so complete, that such of the shore-followers as were 

 unable to adapt themselves to it succumbed, while the others that 

 adapted themselves to the change altered their specific characters 

 altogether. The Nautilida) having come into existence long prior 

 to the introduction of the Ammonitidse, and having also survived 

 the destruction of the latter family, must have possessed in a 

 remarkable degree a power of adapting themselves to altered 

 conditions. It is evident also that ocean-rangers, such as the 

 Dibranchiata, would be independentof those geographical changes; 

 and these, again, are the forms which have been the most com- 

 pletely preserved, and which still exist as an important family*. 



* The effect of geographical configuration upon marine life is shown 



