Fig. 2. — First and last appearances of moUuscan genera in New Zealand during 

 Pliocene and Pleistocene time 



are plotted at the beginning of the interval representing" that stage, 

 and " last appearances " at the end of that interval. The marked 

 inverse relationship between first appearances (= invasion) and last 

 appearances (= extinction) is apparent. The times when large numbers 

 of genera appeared were times when few extinctions occurred, and, 

 conversely, when extinctions were numerous invasions were few. The 

 invasion rate dropped steadily until the Mangapanian, had recovered 

 b}^ the Marahauan, dropped again to introduce the Okehuan, and 

 recovered temporarily at the beginning of the Putikian. The extinction 

 rate rose highest in the two " lean periods." Thus the factors which 

 caused the extinction of moUuscan genera seem to have also inhibited 

 the invasion of others. Conditions favouring a fairly continuous, though 

 decreasing, invasion of genera were interrupted on at least two occasions 

 by the temporary onset of conditions speeding extinction and sub- 

 stantial] v slowing^ invasion. 



31] 



