62 Proceedings of the Roycd Physical Society. 



as were sensitive to changes of this kind, either by the 

 direct influence of the cold, or by its indirect influence upon 

 the plankton which furnished their food-supply, were either 

 killed off, or else escaped extermination by drifting to other 

 areas. Other species, whose fry were not affected by these 

 changes, passed to the adult stage, and took their place at 

 or near the sea-bottom. The descendants of those who were 

 expatriated may very well have been long before they made 

 their way back ; and it is quite conceivable that such forms 

 might, in the meantime, have undergone some modification 

 and have developed into new species — perhaps with different 

 susceptibilities in the matter of cold. 



This explanation appears to me to harmonise well with 

 the facts, and it has the further advantage of calling the 

 attention of palaeontologists, and also of biologists in general, 

 to the importance of taking into fuller consideration than 

 has yet been done the share taken by different temperatures 

 of the ocean surface-water in developing, or retarding, the 

 welfare of many other animals whose earlier life-stages are 

 passed near the surface of the sea. 



I have elsewhere^ suggested that the remarkably wide 

 vertical range of the fauna of the Lower Carboniferous 

 Rocks is capable of explanation on the assumption that the 

 sea-surface varied but little in temperature at any time 

 throughout the whole of that period, and that none of the 

 animals were killed off during the early stages of their 

 existence, as must have been the case had the range been 

 otherwise. 



On the other hand, we may, on the same ground, assume 

 that a similar explanation to that given above in connection 

 with the Proterozoic Graptolites may be applied also to the 

 zonal fossils of the Jurassic Rocks, and especially to the 

 Ammonites. 



It remains for us to consider whether these oscillations of 

 surface temperature can be connected in any way with 

 secular causes, such as the Precession of the Equinoxes. 



1 Trans. Geol. Soc. Glasgow, vol. xii. pp. 34-38 (1902). 



