242 



NATURE 



[October 28, 1915 



the Milne seismograph by Messrs. J. J. Shaw and 

 J. H. Burgess. 



The meeting concluded on Friday afternoon with 

 a very interesting address by Prof. Pierre Weiss on 

 new views of magnetism, in which he described his 

 researches on the part played by the magneton, or 

 definite unit of magnetism, in the phenomena of 

 iron, nickel, cobalt, and their alloys. 



THE BRITISH ASSOCIATION. 

 SECTION K 



Opening Address ^ by Prof. W. H. Lang, F.R.S., 

 President of the Section. 



Phyletic and Causal Morphology. 



I PROPOSE to deal with some aspects of the study 

 of plant-morphology. In doing so I shall not accept 

 any definition of morphology that would separate it 

 artificially from other departments of botany. I re- 

 gard the aim of plant-morphology as the study and 

 scientific explanation of the form, structure, and 

 development of plants. This abandons any sharp 

 separation of morphology and physiology, and claims 

 for morphology a wider scope than has been customary 

 for the past fifty years. During this period the 

 problem of morphology has been recognised as being 

 "a purely historical one," "perfectly distinct from 

 any of the questions with which physiology has to 

 do," its aim being "to reconstruct the evolutionary 

 tree." The limitation of the purpose of morpho- 

 logical study, expressed in these phrases from the 

 admirable addresses delivered to this section by Dr. 

 Scott and Prof. Bower some twenty years ago, was 

 due to the influence of the theory of descent. I fully 

 recognise the interest of the phyletic ideal, but am 

 unable to regard it as the exclusive, or perhaps as 

 the most important, object of morphological investi- 

 gation. To accept the limitation of morphology to 

 genealogical problems is inconsistent with the pro- 

 gress of this branch of study before the acceptance of 

 the theory of descent, and leaves out many of the 

 most important problems that were raised and studied 

 by the earlier mcrphologists. 



In the history of morphology, after it had ceased 

 to be the handmaid of the systematic botany of the 

 higher plants, we may broadly distinguish an ideal- 

 istic period, a developmental period, and a phyletic 

 period. The period of developmental morphology, 

 the most fruitful and the most purely inductive in our 

 science, was characterised by an intimate connection 

 between morphological and physiological work. 

 Among its contributions were studies of development 

 or "growth histories" of whole plants and their 

 members. These were carried out, in part at least, 

 in order to investigate the nature of development, 

 and such general problems found their expression at 

 the close of the period in the "AUgemeine Morpho- 

 logic" of Hofmeister. The "Origin of Species" 

 took some years before it affected the methods and 

 aims of botanical work. Then its effect on morpho- 

 logy was revolutionary, and, as in all revolutions, 

 some of the best elements of the previous regime were 

 temporarily obscured. This excessive influence of the 

 theory of descent upon morphology did not come from 

 Darwin himself, but from his apostle Haeckel, who 

 gave a very precise expression to the idea of a 

 genealogical grouping of animals and plants, illus- 

 trated by elaborate hypothetical phylogenetic trees. 

 Such ideas rapidly dominated morphological work, 

 and we find a special "phylogenetic method" advo- 



1 Abridged by the author. 



NO. 2400, VOL. 96] 



cated by Strasburger. The persistence of the phyletic 

 period to the present time is shown, not only in the 

 devotion of morphology to questions of relationship, 

 but in the attempts made to base homologies upon 

 descent only. Lankester's idea of homogeny can be 

 traced to the influence of Haeckel, and nothing shows 

 the consistency of phyletic morphology to its clear 

 but somewhat narrow ideal so plainly as the repeated 

 attempts to introduce into practice a sharp distinction 

 between homogjny and homoplasy. 



Prof. Bower, in his address last year and in other 

 papers, has dealt illuminatingly with the aims and 

 methods of phyletic morphology. I need only direct 

 attention to some aspects of the present position of 

 this, which bear on causal morphology. The goal of 

 phyletic morphology has throughout been to construct 

 the genealogical tree of the vegetable kingdom. In 

 some ways this seems farther off than ever. Phyletic 

 work has been its own critic, and the phylogeny of 

 the genealogical tree, since that first very complete 

 monophyletic one by Haeckel, affords a clear example 

 of a reduction series. The most recent and trust- 

 worthy graphic representations of the inter-relation- 

 ships of plants look more like a bundle of sticks 

 than a tree. Consider for a moment our complete 

 ignorance of the inter-relationships of the Algae, 

 Bryophyta, and Pteridophyta. Regarding the Algae 

 we have no direct evidence, but the comparative study 

 of existing forms has suggested parallel developments 

 along four or more main lines from different starting- 

 points in a very simple unicellular ancestry. We 

 have no clue, direct or indirect, to the ancestral forms 

 of the Bryophyta, and it is an open question whether 

 there may not be as many parallel series in this group 

 as in the Algae. The Pteridophyta seem a better 

 case, for we have direct evidence from fossil plants 

 as well as the comparison of living forms to assist 

 us. Though palasobotany has added the Spheno- 

 phyllales to the existing groups of vascular crypto- 

 gams and has greatly enlarged our conceptions of 

 the others, there is no proof of how the great groups 

 are related to one another. As in the Bryophyta, they 

 may represent several completely independent parallel 

 lines. There is no evidence as to what sort of plants 

 the Pteridophyta were derived from, and in particular 

 none that relates them to any group of Bryophyta or 

 Algae. I do not want to labour the argument, but 

 much the same can be said of the seed-plants, though 

 there is considerable evidence and fairly general agree- 

 ment as to some Gymnosperms having come from 

 ancient Filicales. The progress of phyletic work has 

 thus brought into relief the limitations of the possible 

 results and the inherent diflficulties. As pointed out 

 by Prof. Bower, we can hope for detailed and definite 

 results only in particularly favourable cases, like that 

 of the Filicales. 



The change of attitude shown in recent phyletic 

 work towards "parallel developments in phyla which 

 are believed to have been of distinct origin " is even 

 more significant. Prof. Bower spoke of the preva- 

 lence of this as an "obstacle to success," and so it is 

 if our aim is purely phyletic. In another way the 

 demonstration of parallel developments constitutes a 

 positive result of great value. Thus Prof. Bower's 

 own work has led to the recognition of a number of 

 series leading from the lower to the higher Filicales. 

 By independent but parallel evolutionary paths, from 

 diverse starting-points in the more ancient ferns, such 

 similarity has been reached that systematists have 

 placed the plants of distinct origin in the same genus. 

 In these progressions a number of characters run 

 more or less clearly parallel, so that the final result 

 appears to be due "to a phj^Ietic drift that may have 

 affected similarly a plurality of lines of descent." 

 This conclusion, based on detailed investigation, 



