No. 596] RAPIDITY OF EVOLUTION 467 



from fifty to one hundred generations a century, most 

 shrubby ones from ten to thirty and most trees only 

 four or five. The degree of variability and other factors 

 being- equal, therefore, one would expect changes to accu- 

 mulate much more rapidly and evolutionary progress 

 consequently to be much faster among herhs than among 

 woody plants. Is there evidence that this is actually the 

 case? 



In an attempt to obtain such evidence an analysis as to 

 growth-habit was first made of the endemic portion of the 

 floras of several regions. Upon the biological isolation 

 of an area the new varieties, species and genera of plants 

 which gradually take their origin in its flora are neces- 

 sarily limited in their distribution to the region in ques- 

 tion, or are "endemic" in it; and these local types will 

 evidently be produced first and in greatest abundance by 

 those elements in the flora which are changing most 

 rapidly. Consequently that particular growth type which 

 is found to predominate among such endemic forms may 

 justly be regarded as the one whose members are under- 

 going the most rapid alteration. In an analysis of the 



used to distinguish carefully two radically different types 

 of endemic plants: those under discussion, which were 

 local in origin and have never spread abroad; as con- 

 trasted with those which owe their present localization 

 rather to the fact that they are isolated survivors of 

 genera or species at one time much more widely distrib- 

 uted. The former category, which we may call the "in- 

 digenous" endemics, will evidently represent a new ele- 

 ment in the flora; the latter, or * ' relict' ' endemics, a very 

 old one. It is not always easy to separate sharply these 

 two types in a given flora, but species, and especially 

 genera, which stand apart and possess no near relatives 

 in the region are, in most cases, at least, evidently to be 

 looked upon as relicts ; whereas in a group of species or 

 genera the members of which are numerous and closely 

 related one to another we doubtless behold a body of 

 plants undergoing evolutionary development on the spot. 



