October 28, 19 15] 



NATURE 



243 



.appears to me to be of far-reaching importance. If 

 la **phyletic drift" in the ferns has resulted in the 

 independent and parallel origin of such characters as 

 iictyostely, the mixed sorus, and the very definite type 

 )f sporangium with a vertical annulus and transverse 

 lehiscence, the case for parallel developments in other 

 groups is greatly strengthened. The interest shifts 

 the causes underlying such progressive changes 

 IS appear in parallel developments, and the problem 

 »ecomes one of causal morphology rather than purely 

 listqrical. 



The study of parallel developments would, indeed, 

 seem likely to throw more light on the morphology 

 plants than the changes traced in a pure phyletic 

 me, for it leads us to seek for common causes, 

 R'hether internal or external. We cease to be limited 

 in our comparisons by actual relationship, or for- 

 bidden to elucidate the organisation in one group by 

 that which has arisen indef>endently in another, 

 limilarly the prohibition against comparing the one 

 generation in the life-cycle with the other falls to the 

 ground, quite apart from any question of whether 

 tlir alternation is homologous or antithetic. The 

 methods of advance and the causal factors concerned 

 tifcome the important things, and if, for example, 

 lii^ht is thrown on the organisation of the fern-plant 

 b\ comparison with the gametophyte of the moss, so 

 much the better. This, however, is frankly to 

 abandon ph3iogeny as "the only real basis of morpho- 

 logical study," and with this any attempt to laase 

 homology on homogeny. Many of the homologies 

 [that exist between series of parallel development are 

 |-hat have been happily termed homologies of organ- 

 isation ; these are sometimes so close as to result in 

 -actical identity, at other times so distinct as to be 

 ivident homoplasies. The critical study of homo- 

 >gies of organisation over as wide an area as possible 

 »mes of primary interest and importance. 

 Since about the beginning of the present century 

 change of attitude towards morphological problems 

 IS become more and more evident in several ways. 

 It seems to be a phyletic drift affecting simultaneously 

 plurality of lines of thought. The increasing ten- 

 ancy to look upon problems of development and 

 instruction from a causal point of view is seen in the 

 •minence given to what may be termed develop- 

 lental physiologv. and also in what Goebel has called 

 )rganography. These deal with the same problems 

 Trom different sides and neither formulates them as 

 they appear to the morphologist. Together with 

 [^enetics, they indicate the need of recognising what 

 prefer to call general or causal morphology. 

 The problems of causal morphology are not new, 

 lOugh most of them are still unsolved and are difTi- 

 ilt to formulate, let alone to answer As we have 

 sn, they were recognised in the time of develop- 

 sntal morphology, though they have since been 

 nost wholly neglected by morphologists. So far 

 they have been studied during the phyletic period, 

 has been from the physiological rather than the 

 •morphological side. Still, such problems force them- 

 ives upon the ordinary morphologist, and it is from 

 - position thiit I venture to approach them. I 

 illingly recognise, however, that causal morphology 

 Pi.iy also be regarded as a department of plant- 

 livsiology. In development, which is the essential 

 the problem, the distinction between morphology 

 1 physiology really disappears, even if this distinc- 

 iHui can be usefully maintained in the study of the 

 fully developed organism. We are brought up against 

 a fact which is readily overlooked in these days of 

 ■ cialisation, that botany is the scientific studv of 



MltS. 



'reneral morphology agrees with physiology in its 

 aim. being a causal explanation of the plant and not 

 NO. 2400, VOL. 96] 



historical. Its problems would remain if the phyletic 

 history were before us in full. In the present state 

 of our ignorance, however, we need not be limited to 

 a physico-chemical explanation of the plant. Modern 

 physiology rightly aims at this so far as possible, but, 

 while successful in some departments, has to adopt 

 other methods of explanation and analysis in dealing 

 with lirritability. It is even more obvious that no 

 physico-chemical explanation extends far enough to 

 reach the problems of development and morphological 

 construction. The morphologist must therefore take 

 the complicated form and its genesis in development 

 and strive for a morphological analysis of the 

 developing plant. This is to attack the problem from 

 the other side, and to work back from the phenomena 

 of organisation toward concepts of the nature of the 

 underlying substance. ; • 



It is to these questions of general morphology with 

 a causal aim (for causal morphology, though con- 

 venient, is really too ambitious a name for anything 

 we yet possess) that I wish to ask your attention. 

 All we can do at first is to take up a new attitude 

 towards our problems, and to gather here and there 

 hints upon which new lines of attack may be based. 

 This new attitude is, however, as I have pointed out, 

 a very old one, and in adopting it we re-connect with 

 the period of developmental morphology. Since the 

 limited time at my disposal forbids adequate refer- 

 ence to historical details, and to the work and thought 

 of many botanists in this field, let me in a word dis- 

 claim any originality in trying to express in relation 

 to some morphological problems what seems to mc 

 the significant trend, in part deliberate and in part 

 unconscious, of morphology at present. The methods 

 available in causal morphology are the detailed study 

 in selected plants of the normal development and its 

 results, comparison over as wide an area as possible, 

 with special attention to the essential correspondences 

 (homologies of organisation) arrived at independently, 

 the study of variations, mutations, and abnormalities 

 in the light of their development, and ultimately 

 critical experimental work. This will be evident in 

 the following attempt to look at some old questions 

 from the causal point of view. I shall take them as 

 suggested by the fern, without confining my remarks 

 to this. The fern presents all the main problems in 

 the morphology of the vegetative organs of th.- 

 higher plants, and what little I have to say regard- 

 ing the further step to the seed-habit will come as a 

 natural appendix to its consideration. 



Individual Development. 



Twice in its normal life-history the fern exhibits a 

 process of development starting from the single cell 

 and resulting in the one case in the prothallus and 

 in the other in the fern-plant For the present we 

 may treat these two stages in the life-history as 

 individuals, their development presenting the same 

 general problems as a plant of, say, Fucus or Entero- 

 morpha, where there is no alternation of generations. 

 How is the morphologist to regard this process of 

 individual development ? 



In the first place, we seem forced to regard the 

 specific distinctness as holding for the germ as well 

 as the resulting mature plant, however the relation 

 between the germ-cell and the characters of the 

 developed organism is to be explained. We start 

 thus with a conception of specific substance, leaving 

 it quite an open question on what the specific nature 

 depends. This enables us to state the problem of 

 development freed from all considerations of the ulti- 

 mate uses of the developed structure. The course of 

 development to the adult condition can be looked 

 upon as the manifestation of the properties of the 

 specific substance under certain conditions. This 



