2 THE PROBLEMS OF EVOLUTION 



time it has been necessary to devote a considerable 

 amount of effort to the adequate estabhshment of 

 the fact of its occurrence, but the method of evolu- 

 tion has also been the subject of extensive discus- 

 sion. The matter has been handled controversially 

 to such a degree that expressions of opinion are 

 much more plentiful than sound discussions, but 

 here and there in the literature, especially in re- 

 cent years, a new and valuable trend in the discus- 

 sion of phylogenetic processes may be noted. Ap- 

 parently our knowledge has reached a point where 

 rational analysis is being diligently attempted. 



The expressed conclusions of writers on this 

 subject are so different that they can hardly re- 

 sult in an early agreement, but they strike at the 

 heart of existing difficulties in so many cases that 

 the prospect of progress is bright. We have been 

 concerned for years with the gathering of new data 

 and have clung so tenaciously to the old theories 

 and the old theoretical controversies that any 

 departure from the established mode of treatment 

 is promising, granting only that it partakes of an 

 open-minded point of view and does not introduce 

 a new bias among those already existing. Such a 

 work as this cannot pretend to be an ultimate solu- 

 tion, any more than the contributions of many 

 other scientists, but the time seems ripe for a 

 soundly logical analysis of the problems of phylo- 

 genesis and this is an attempt at such analysis. In 



