98 FREDERICK ORPEN BOWER 



The first datum is the presence living upon the earth of individ- 

 uals illustrating numerous plant forms which appear distinct by 

 character and by genesis. A comparison of those plant forms one 

 with another shows underlying similarities. Where these are 

 greater than their points of difference they are held to be indi- 

 cations of natural affinity, and a general theory of evolution based 

 on such comparisons has long been entertained as accounting for 

 their origin. But it is necessary to remember always that this 

 is not, and probably never can be more than a working hypothesis. 

 Secondly, within narrow limits of variation like produces like. 

 Hence the inference is justified that, in broad lines, what holds for 

 the present has also been the rule for the past. It is accordingly 

 a general view that evolution has been slow, and has progressed 

 by relatively gradual steps of divergence of offspring from the pro- 

 genitors. Thirdly, though certain of the forms we find now liv- 

 ing graduate by very gentle steps one into another, and thus form 

 natural groups or families, others are dissociated and relatively 

 isolated. In extreme cases a genus such as Isoetes, or a single 

 species such as Welwitschia mirabilis is thus recognized as mono - 

 typic. Such forms provide the most obvious problems of evolu- 

 tion, though they are also the most difficult of solution. For their 

 very isolation, suggests that many of the gradual evolutionary 

 steps have been lost, and that the sequence of living forms reflects 

 the past history very imperfectly. Fourthly, the geological strata 

 of successive ages have yielded evidence of a succession of fossil 

 forms, comparable in varying degree with those of the living flora. 

 From these some positive indication has been derived as to the 

 relative ages of certain of the living types. These indications 

 serve as suggestions as well as checks upon the efforts of compara- 

 tive morphology — that branch of the science which proceeds pri- 

 marily from the study of organisms now living. 



It must be obvious to those who study either living forms or 

 fossils, or what is better, to those who study both, that the record 

 from either source, or from both combined, is very incomplete. 

 The frequent existence of monotypic forms is a pregnant fact, and 

 still more their prevalence among those which comparison, as 

 well as the fossil record, teaches us were relatively early. Thus 



