INTRODUCTION TO VOLUME II 9 



possibly follow naturally in any simple linear sequence. The succession in 

 which they are taken up must therefore be somewhat artificial. Short series 

 may be suggested, and built up. But the actual evolutionary relation of 

 those series to one another will still remain highly problematical. The time 

 has not yet come for any confident reconstruction of an "evolutionary tree," 

 which shall represent the phylesis of the Filicales as a series of connected 

 branches springing from a common trunk, even if there ever was one. 

 Nevertheless the relative position of those branches is a legitimate subject 

 for discussion, and definite views may even be entertained regarding it. 



In any Family of related organisms, excepting the very simplest, diffi- 

 culties must be expected in grouping them phyletically. The more complex 

 the organisms are, and the more numerous the criteria of their comparison, 

 the greater those difficulties may be expected to be: for there is no reason 

 to anticipate that evolution will have expressed itself equally in respect of 

 all the features. x\nd so it comes about that it appears to be sometimes 

 impossible to sum up the results of comparison in respect of all the features 

 into a conclusion, that genus or species (A) is more primitive than genus or 

 species (B). Such difficulties naturally present themselves when any attempt 

 is made to seriate any group of organisms phyletically: and any one who 

 fully grasps the problem will place his own value upon the phyletic seria- 

 tion of any group submitted for his judgment. Some may say, "If that is 

 true, why attempt phyletic seriation at all?' The proper reply to that ques- 

 tion is, "Why study Evolution at all?" Phyletic seriation is merely a form 

 of recording tentative evolutionary conclusions: it has, however, the advan- 

 tage of raising questions which otherwise might very likely remain dormant. 



