L] QUESTION OF CAUSALITY 293 



or from without: or is it some interaction of both which produces the in- 

 herited result?^ The existence of so wide a common trend of change as is 

 seen in the parallel progressions of Ferns, in respect of the several criteria 

 of comparison not only individually but also collectively, is a fact that 

 strongly suggests some defining tendenc}- within the organism, promoting 

 its development not only in individual features, but also as a whole. Modi- 

 fication or even stimulus due to the incidence of external conditions, however 

 effective, does not suffice to give a full explanation of so broad a sweep of 

 cognate change as that from the Eusporangiate to the Leptosporangiate 

 type. The facts appear to suggest for the Filicales not only detailed modifica- 

 tion influenced from without, but also some more general bias or tendency 

 of initiative within the organism itself: not fortuitous, nor yet a vague muta- 

 bility in all directions, as De Vries would suggest, though this need not 

 necessarily be ruled out: but specifically directive, upon the results of which 

 limiting factors, whether environmental or internal, have acted in defining 

 and shaping the heritable details. Such a conception, prompted as it is by 

 intensive study of a wide area of fact, may go far towards explaining that 

 majestic and highly polyphyletic progression which leads from the Palaeozoic 

 Coenopteridaceae to the Leptosporangiate Ferns of the Present Day. Such 

 a general tendency of initiative applicable for the great Class of the Filicales 

 would suggest a still wider and more varied application in the Evolution of 

 Organic Life at large. 



If such a tendency be held as included in "the nature of the organism," 

 and the results are hereditary as the evidence from the Ferns indicates that 

 they are, then this position appears to be in accord with the expressions of 

 Darwin, in Chapter V of the Origin of Species, where he says: "In all cases 

 there are two factors, the nature of the organism, which is much the most 

 important of the two, and the nature of the conditions. The direct action of 

 changed conditions leads to definite or indefinite results. In the latter case 

 the organisation seems to become plastic and we have much fluctuating 

 variability. In the former case the nature of the organism is such that it 

 yields readily, when subjected to certain conditions, and all or nearl}' all 

 the individuals become modified in the same way." 



^ The argument stated in this Chapter accords with the views so well expressed by Vines, in his 

 Article on "Morphology of Plants" in the Encyclopaedia Britannica, nth edn. 1910-11: "In 

 endeavouring to trace the causation of adaptation, it is obvious that it must be due quite as much to 

 properties inherent in the plant as to the action of external conditions; the plant must possess adaptive 

 capacity. In other words, the plant must be irritable to the stimulus exerted from without, and be 

 capable of responding to it by changes of form and structure. Thus there is no essential difference 

 between the 'direct' and the 'indirect' action of external conditions, the difference is one of degree 

 only. In the one case the stimulus induces indefinite variation, in the other definite; but no hard-and- 

 fast line can be drawn between them." Inasmuch as specific examples are now given, showing the 

 establishment of definite adjustments, in the first instance ontogenetic, as inherited characters, the view 

 of Vines is thereby advanced from the phase of theoretical discussion towards that of demonstration. 



D. H. HILL LIBRARY 



