No. 596] 



RAPIDITY OF EVOLUTION 



475 



A study of the comparative rate of evolution among the 

 Angiosperms also throws a little light on tin- aiiii<|uity of 

 this great group. Its fossil representatives have as yet 

 failed to make their appearance in strata lower than t In- 

 lowest Cretaceous, and it is commonly assumed that t hex- 

 had their origin at about that period. We have already 

 noted the probability that they were represented at their 

 inception only by woody forms. There is evidence that 

 the herbaceous type, save perhaps as a negligible portion 

 of the flora, did not arise till the early Tertiary, since 

 which time its members have undergone practically their 

 entire evolutionary development. As to the relative 

 lengths of geological periods there is no very definite 

 evidence, but authorities agree that the Mesozoic was of 

 considerably longer duration than the Tertiary, a com 

 servative estimate placing the former at about three 

 times the length of the latter. Let us grant for purposes 

 of argument that this is a reasonably close approach to 

 the truth ; and let us also assume that herbs have evolved 

 at a rate averaging twice as great as that of woody plants, 

 surely a moderate supposition. All the 2,600 exclusively 

 herbaceous genera presumably developed as herbs; and 

 there are 4,200 genera which contain woody plants and 

 which probably originated as woody plants. Now if these 

 2,600 herbaceous genera have been developed since the 

 beginning of the Tertiary, woody plants, producing new- 

 forms at half this rate, would during the same time have 

 given rise,letus say, to 1,300 genera. To produce the other 

 2,900 woody genera (to say nothing of the hundreds wlm-h 

 have become extinct) would require, assuming the same 

 rate of evolution, another period twice as long as the I <-r 

 tiary, thus thrusting back the origin of the Angio^crms 

 to a date distant from the present thrice the length mi !,«■ 

 Tertiary. According to the estimate mentioned above-, 

 this would be about the beginning of the Jurassic. Ot 

 course this is all extremely hypothetical, but it serves to 

 emphasize the probability that in order to have developed 

 their great number and diversity of slowly changing 



