Septembeb llj 1914] 



SCIENCE 



369 



in the prevalence of parallel development 

 in phyla which are believed to have been 

 of distinct origin. This is exemplified very 

 freely in the ferns, and the systematist has 

 frequently been taken in by the resem- 

 blances which result from it. He has 

 grouped the plants which show certain 

 common characters together as members of 

 a single genus. Sir William Hooker in 

 doing this merged many genera of earlier 

 writers. His avowed object was not so 

 much to secure natural affinity in his sys- 

 tem as readiness of identification : and con- 

 sequently in the "Sjmopsis Filicum" there 

 are nominal genera which are not genera 

 in the phyletic sense at all. For instance, 

 Polypodium and Acrostichum, as there de- 

 fined, may be held from a phyletic point of 

 view to be collective groupings of all such 

 ferns as have attained a certain state of 

 development of their sorus; and that they 

 are not true genera in the sense of being 

 associated by any kinship of descent: this 

 is shown by the collective characters of the 

 plants as a whole. Already at least four 

 different phyletic sources of the Acrostic- 

 hoid condition have been recognized, and 

 probably the sources of the Polypodioid 

 condition are no fewer. Such "genera" 

 represent the results of a phyletic drift, 

 which may have affected similarly a plural- 

 ity of lines of descent. It will be the 

 province of the systematist who aims at a 

 true grouping according to descent to comb 

 out these aggregations of species into their 

 true relationships. This is to be done by 

 the use of wider, and it may be quite new 

 criteria of comparison. Advances are be- 

 ing made in this direction, but we are only 

 as yet at the beginning of the construction 

 of a true phyletic grouping of the Filicales. 

 The more primitive lines are becoming 

 clearer: but the difficulty will be greatest 

 with the distal branches of the tree. For 

 these represent essentially the modem 



forms, they comprise the largest number 

 of apparently similar species, and in them 

 parallel development has been most preva- 

 lent. 



If this difficulty be found in such a 

 group as the Filicales, in which the earlier 

 steps are so clearly indicated by the re- 

 lated fossils, what are we to say for the 

 Angiosperms? Our knowledge of their 

 fossil progenitors is very fragmentary. 

 But they are represented now by a multi- 

 tude of forms, showing in most of their 

 features an irritating sameness. For in- 

 stance, vascular anatomy, that great re- 

 source of phyletic study in the more primi- 

 tive types, has sunk in the Angiosperms 

 to something like a dead level of uni- 

 formity. There is little variety found in 

 the contents of embryo-sacs, in the details 

 of fertilization, or in embryology. Even 

 the ontogeny as shown in the seedling 

 stages affords little consolation to the 

 seeker after recapitulation. On the other 

 hand, within what are clearly natural 

 circles of affinity there is evidence of an 

 extraordinary readiness of adaptability in 

 form and structure. Such conditions sug- 

 gest that we see on the one hand the far- 

 reaching results of parallel development, 

 and on the other the effects of great 

 plasticity at the present day, or in rela- 

 tively recent times. Both of these are 

 points which prevent the ready tracing of 

 phyletic lines. In the absence of reliable 

 suggestions from paleontology, the natural 

 consequence is the current state of uncer- 

 tainty as to the phyletic relations of the 

 Angiosperms. 



Various attempts have been or are being 

 made to meet the difficulty. Some, on the 

 basis of the recent observations of Wieland 

 and others, are attempting along more or 

 less definite monophyletic lines to con- 

 struct, rather by forcible deduction than 

 by any scientific method of induction, an 



