xM PKOCEEDINGS OF THE GEOLOGICAL SOCIETY. 



is of such a kind that one absorbs the carbonic acid which the other 

 rejects, one restores the oxygen which the other takes awaj< . This 

 reciprocity, considered in regard to the sea, is more important than 

 in respect of the free air. Without marine plants it is almost incon- 

 ceivable that the respiration of marine animals would be provided 

 for, especially in the deeper and more tranquil parts of the water. 

 We may consistently believe — and observation appears to justify the 

 belief — that in no period within the reach of Palaeontology were 

 marine plants wanting, except in the parts of the ocean so deep that 

 air and light and motion are very feeble in their influence. In 

 marine plants we have one long series of very analogous forms from 

 the earliest periods, corresponding to the uniformity of the conditions 

 of their life. Another law very frequently traceable in existing nature 

 is found in the reciprocity of herbivorous and carnivorous races, 

 by which, in connexion with plants, a complete circle of growth and 

 sustenance is established on the earth and in the water — commencing 

 with the atmosphere as the food of plants, these contributing to the 

 nourishment of half the animal world, which in its turn sustains 

 the flesh-eating races. This dependence is of such an order that the 

 herbivorous races may be conceived to exist without any carnivora, 

 but not the converse, and that one race of herbivora may be 

 balanced by one or another or a mixture of several carnivorous tribes 

 suited to the same element. In looking at the scale of ancient life 

 in the sea we shall find it preferable, in the first instance, to sejmrate 

 the tribes which, like the Brachiopoda and Lamellibranchiata, are 

 nourished by Infusoria coming to their feeding-organs with currents 

 of water. So simple and easy a nutrition seems to warrant the 

 expectation that they were less liable to great variation of type than 

 carnivorous and even herbivorous races, whose life is more varied 

 and more dependent on the changes of external conditions; Let 

 us consider these points in regard to the marine races whose remains 

 are found fossil. Few tribes offer in this respect a more remarkable 

 contrast than the Brachiopoda as Infusorial feeders, and the Gastero- 

 poda, which as a class may be regarded as one -half herbivorous and 

 one-half carnivorous*. The Brachiopoda counted by genera diminish 

 almost regularly with the lapse of time, from 18 Silurian genera 

 to 2 Eocene ; while the Gasteropoda commence with 18 Silurian, and 

 augment to 78 or more Eocene. Again, if we separate the Gaste- 

 ropoda into two groups, the Herbivora commence with 18 Lower 

 Palaeozoic, and go on augmenting to above 50 Eocene genera, while 

 the Carnivora are almost unknown in all the Palaeozoic groups, but 

 grow continually in number to nearly 30 of Eocene date. These 

 and some other relations appear in the following Table, which shows 

 further that Monomyaria and Cephalopoda attain a maximum of 

 generic variety in the Mesozoic strata : — 



* In the following remarks, the Holostomatous Gasteropoda are counted as 

 herbivorous, the Siphonostoinatous genera are included as carnivorous: this 

 classification, though not strictly correct, is the only one applicable to the present 

 purpose. 



