144 AGE AND AREA FROM A [pt. ii 



more common for change to have occurred, and we find that 

 the greater the lapse of time, the greater proportionately has 

 been the change. That is to say, more species found in the older 

 deposits are extinct, than in the newer. This may very clearly 

 be seen by comparing the percentages of species and varieties, 

 which there is reason to think are extinct, in the successive 

 Pliocene floras. There is an element of uncertainty in such a 

 comparison for this reason. It has not always been possible, for 

 lack of living material, to discover whether a seed belongs to a 

 living species or not. The following figures will, I believe, have 

 at least some approximation to the truth. The deposits read 

 downwards in order of age ; they are those from which the main 

 evidence of the facts discussed in this paper were derived. 



Percentages of extinct species belonging to the Chinese- North- 

 American Association of Plants in the West European Pliocene 

 at successive periods. 



The figures show clearly a progressiA^e extermination of older 

 forms as compared with newer. In some way age has acted as 

 an exterminating agent. Hoav? Is it, to use the language of Age 

 and Area, by dying out, or by killing out? 



I take it that, by the use of these terms, Dr Willis intends to 

 distinguish between extinction of species due to exhaustion of 

 vitality, and extinction of species due to external agencies. That 

 is, between internal and external causes. It cannot always be 

 possible to distinguish between these two, for frequently, as we 

 know, external causes are assisted in their work of destruction 

 by pre-disposing conditions in the individual, or it may be in the 

 race. Such relationships are seen in that of disease to sus- 

 ceptibility to disease, of change of climate to a Aveakened con- 

 stitution. It is the external cause which seems the cause of 

 death, though the internal cause may have an equal share in it. 



Viewing the fact of the immense amount of extermination 

 that has occurred in this flora, I was at first inclined to think 

 that such destruction of species, one might almost call it uni- 



