22 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



and higher development, with special adaptation to resist 

 violent changes of temperature, etc. 



In his opening address to Section C (Geology) at the 

 British Association, Cardiff, 1920, Dr. F. A. Bather quotes 

 Professor Abel as maintaining that the varying " tempo " of 

 evolution in the case of the sirenians and horses of the Tertiary 

 can be correlated with the food-supply. The sirenians under- 

 went a steady, slow change because although they migrated 

 from land to sea, they retained the habit of feeding on soft 

 water-plants. Horses, though remaining on land, evolved at 

 first rapidly, and then more slowly, but up to Pliocene times 

 always more quickly than the sirenians. Such evolution might 

 be correlated with their change into eaters of grain, and their 

 adaptation to a life on the plains where food of this character 

 was available. The whales, like the sirenians, migrated at the 

 beginning of Tertiary time from the land to the sea. But their 

 rate of evolution was altogether different, hence very diverse 

 forms resulted. At first they remained near the coasts, and 

 kept to the ancestral diet, with consequent slow change. 

 Then they took to hunting fish, and afterwards to eating 

 cephalopods ; from Oligocene times onward the change was 

 thus very rapid, and a great burst in evolution in Miocene 

 times resulted. Finally, many turned to minute floating 

 organisms as food, and from Lower Pliocene times to the present 

 day the change has been very slow. 



Dr. Bather rightly emphasizes the fact that any attempts 

 to frame a causal connexion are bound to be speculative. 



Numerous cases can be quoted from the geological record 

 of the gradual changes hi a fauna resulting from a gradually- 

 changing environment. Two parallel examples may be 

 mentioned of the effect produced on molluscan life by a gradual 

 elevation of the sea-bed sufficient to cut off arms of the sea 

 and produce lakes which continued to shrink. In the first 



