sinnott: endemism as a criterion of antiquity 165 



which we beHeve to have arisen for the most part since the isola- 

 tion of America from Europe, been extended much further and 

 developed into new genera? 



The explanation of the whole matter apparently lies in the fact 

 that herbs, because of the brevity of their life cycle, are subject 

 to much more rapid evolutionary change than are most woody 

 plants. Given an equal degree of mutability, a species which has 

 a hundred generations a century, as does an annual herb, will 

 accumulate changes much more quickly, and will thus become 

 altered in type much sooner, than will a species having only three 

 or four, as do many trees, or even fifteen or twenty, as do the 

 more rapidly maturing shrubs. 



Other things being equal, therefore, the herbaceous element 

 in any flora is the one which is quickest to change and which is 

 always the first to show the effects of isolation by developing 

 local types. North America and Europe, which have not long 

 been separated from one another, will consequently show many 

 "indigenous" endemic genera which are herbs, but few or none 

 which are trees or shrubs. 



The fact that herbs are so rare among the endemic types of 

 the great land masses of the south temperate zone, as we noted 

 above, is excellent evidence that the herbaceous element in the 

 vegetation of these regions has but recently appeared. Had 

 herbs been a prominent part of the flora there as long as they have 

 in the north, it is hard to believe that they would not likewise 

 have given rise to an endemic element very numerous in genera 

 and species. 



In any discussion of endemism and its usefulness in determining 

 the comparative antiquity of the various portions of a flora, one 

 should therefore make a clear distinction not only between "relict" 

 endemics, which from their nature are among the older members 

 of the flora, and "indigenous" endemics, which may or may not 

 be so; but in addition, and more particularly, between endemic 

 woody plants and endemic herbs. The former, from the extreme 

 slowness with which they tend to become altered in type, may be 

 generally counted upon as the most ancient floral element. Herbs 

 are subject to such rapid evolutionary change that endemic types 

 developed among them cannot well be compared as to antiquity 

 with those appearing among trees or shrubs. 



