128 MARINE BIOLOGICAL LABORATORY. 



their lifetime, a pair of elephants contribute but ten 

 or a dozen young to the race. 



Wallace shows that we are living now in a zoologically 

 impoverished world. Almost all of the largest and 

 strangest forms have recently become extinct : in 

 Europe the great Irish elk, the sabre-toothed tiger, 

 cave-lion, rhinoceros, hippopotamus, and elephant ; in 

 North America equally large felines, horses, and tapirs 

 larger than any now living, a llama as large as a camel, 

 mastodons and elephants, besides a large number of 

 huge megatherians ; in South America an even greater 

 number of megatherians, huge armadillos, a mastodon, 

 large horses and tapirs, large porcupines, two kinds of 

 antelopes, numerous bears and felines, beside the sabre- 

 toothed tiger. 



Remains of all these are found in the recent de- 

 posits, and these animals lived till shortly before the 

 northern continents were encased with the ice of the 

 glacial epoch. It is possible that a change of climate, 

 due to the growing cold from the encroaching ice-belt, 

 affected the flora. This would, of course, affect the 

 food supply, and so tend to lessen the reproductive 

 powers, and shorten the lives of the individuals. Les- 

 sened reproductive power and shortened lives of the 

 individuals would surely result in the extinction of the 

 race, and in this way the destruction of these forms 

 may be accounted for. The starvation of the individual 

 is therefore synonymous with the starvation of the race. 



The main features of Weismann's theory may be 

 summed up as follows : First, The continuity of the 

 germ-plasm. Second, Variation is due to the different 

 molecular combinations formed in the mixture of the 



