114 



NA TURE 



[December i, iJ 



to apply a terminology to the sequences which we recognise, 

 we must, I think, accept phylogeny as the natural basis for 

 morphology. I do not think that any middle course between 

 this and an artificial system is possible or reasonable. But here 

 we launch ourselves upon a sea of uncertainties on which we 

 must keep our course with care. Following it, we think we 

 espy certain great movements in nature. We may recognise 

 what we believe to be a true evolutionary sequence, but who is 

 to say whether it is a progressive or a retrograde sequence? It 

 may be even one divergent from some middle point. Our best 

 friend may read the sequence in opposite order to ourselves and 

 arrive at a diametrically opposite conclusion. There is no 

 finality to this judging of probabilities, a fact which should be 

 always before the mind, especially in the warmer moments of 

 discussion. 



It is interesting to trace the parallel between the progress of 

 classification of plants as a whole, and that of the classification 

 of their parts. In each case the earlier systems were artificial. 

 We may compare the Linnaean system of taxonomy with the 

 Hofmeisterian organography : in both the rigid application of a 

 preconceived method placed incongruous things in juxtaposition, 

 in each case a widening of the basis of the classification has 

 resulted in a redistribution on more natural lines. The present 

 ideal of taxonomy is the same as that of the phylogenetic 

 organography, viz. to group according to descent. The 

 limitations are alike : systematists and morphologists both find 

 their greatest difficulty in the incompleteness of the record, and 

 the frequent isolation of the thing to be classified. 



But without following the obvious parallel further, we may 

 now briefly review our position as regards organography, and 

 the following categories are to be recognissed, though they 

 graduate almost imperceptibly into one another : — ■ 



Homoi^cny. — (a) Repetition of the individual part in suc- 

 cessive generations, with the same number and position. This 

 is exemplified by the cotyledons, the foot, and first root. 



(b) Essential <orresponden<e of parts varying in number and 

 position, but corresponding in character and development, 

 produced in a regular sequence ; e.g. most cases of continued 

 embryology. 



(it) Transferred position of parts, similar in origin and 

 structure to those produced in regular sequence ; e.g. roots, 

 adventitious buds, sori of Aspidiutn aiiomaluut^ aposporous 

 and apogamous growths, many monstrosities ; these we may 

 believe to result from a transfer of inherited developmental 

 capability. 



Homoplasy. — This may be recogni.sed with varying degrees of 

 probability ; starting from cases » here the question of community 

 of descent is open (as with nearer circles of affinity), and pro- 

 ceeding to those in which distinct evolution is virtually certain. 

 It remains for future investigation to clear up doubtful points. 

 Meanwhile, taking the case of leaves for the purpose of illustra- 

 tion, we may conteinplate the following possibilities : — 



{a) A possible origin of two homoplastic series of leaves in 

 the same plant, and the same generation {Phylloglossnin). 



{6} Two homoplastic series in the same plant, but in different 

 generations {LycopoJium cernuuin). 



(c) A possible distinct origin of homoplastic leaves in distinct 

 phyla, but in the same generation (sporophyte of Ferns, 

 Lycopods, Equiseta). 



(tf) A distinct origin of homoplastic leaves in distinct phyla, 

 and distinct generations (e.g. leaves of Bryophyta and of 

 Pteridophyta). 



Now //ontology has been used in an extended sense as 

 including many, or even all, of these categories. It seems plain 

 to me that this collective use of the term homology carries no 

 distinct evolutionary idea with it ; it indicates little more than 

 a vague similarity ; the word will have to be either more strictly 

 defined or dropped. The old categories of parts based upon the 

 place and mode of their origin are apt to be split up if the system 

 be checked by views as to descent. Comparison, aided by 

 experiment, supersedes all other methods, and the results which 

 follow raise the question of terminology of parts which have 

 arisen by parallel development. 



In parts which are of secondary importance, such as stipules, 

 pinnae, the indusium, hairs, glands, the inconstancy of their 

 occurrence points to independent origin by parallel development 

 in a high degree ; in parts of greater importance, such as 

 leaves, a parallel development may also be recognised, though 

 in a less high degree ; in the case of sporangia their acceptance 



NO. 1518, VOL. 59] 



as a category siii generis dispelled the old view of their various 

 origin from vegetative parts ; but we must remember that this 

 does not by any means exclude a parallel development also in 

 them, by enlargement and septation from some simpler spore- 

 producing body, though this is not yet a matter of demonstration. 

 Finally, the sexual organs are probably homogenetic in all 

 Archegoniate plants, but we have no proof that sexuality arose 

 once for all in the lower plants ; the probability is rather the con- 

 trary. Thus we may contemplate as very general a polyphyletic 

 origin of similar parts by evolution along distinct lines, but 

 resulting, it may be, in forms essentially similar. 



There are two extreme courses open to those who wish to 

 convey clearly to others such matters as these ; the one is to 

 use a separate term for each category of parts, which can be 

 followed as maintaining its individual or essential identity 

 throughout a recognised line of descent — in fact, to make a 

 polynoinic terminology of members run parallel with a poly- 

 phyletic development. The other course is to make it clear 

 always in the use of terms applied to parts, that they do not 

 convey any evolutionary meaning, and to use them only in a 

 descriptive sense. Perhaps the former is the ideal method, and 

 it may be a desirable thing, as polyphyletic origins of parts be- 

 come more established, that the terminology should be brought 

 to reflect at least the more important conclusions arriveil at. 

 How this may be done we leave for the future to decide, though 

 I have indicated a first step in the case of the leaves of Mosses 

 and Ferns. 



But, for the present, the whole matter is still so tentative that 

 it is well to be content with something which falls short of the 

 ideal, and to maintain the usual terms, such as stem, leaf, root, 

 hair, sporangium, iS:c. , as simply descriptive of parts which 

 correspond as regards general features of origin, position, and 

 nature ; but with no reference either, on the one hand, to con- 

 formity to any ideal plan, or, on the other, to any community 

 by descent — in fact we shall preserve the original pre- Darwinian 

 sense of these words, which was purely descriptive, and avoid 

 any attempt to read into them any accessory meaning. 



A special interest attends those cases of I ransfer of inherited 

 developmental capability where a part appears with its normal 

 characters, but in a position which is not usual, such as the 

 transfer of the sori of Aspidium aiiomaluin ; comparable with 

 these transfers on the one hand are those apogamous growths 

 where roots, leaves, ramenta, sporangia may arise independently 

 out of the lusual succession. These may be compared, on the 

 other hand, with those interpolations of extra parts, such as 

 the accessory stipules in the stellate Rubiaceae, the extra stamens 

 in Rosaceae, &c. We are unable as yet to say what it is which 

 determines the position and mode of origin of parts ; I do not 

 myself think that Sachs's hypothesis of " Stoff and Form," 

 involving ideas of material differences which have not been 

 demonstrated, will advance the question so much as a careful 

 following of the details in the origin of the parts, say in some 

 of these apogamous growths. Here we see the plant body in a 

 sense analysed before us ; any one part may be produced 

 separately I'rom any other. An elucidation of how any one of 

 these is initiated and determined should lead to a knowledge of 

 the influences which act also in the normal sequence, and 

 determine the origin of pans in the plant body at large. 



I '.lave attempted to touch upon some of those questions in 

 the Morphology of Plants which specially interest us at present, 

 and I dare say in doing so have revealed to you some of the 

 special weaknesses of this branch of the science. The w.ant of 

 finality in this unravelling of history without documents, the 

 ample latitude for difference of opinion, according to the 

 relative weight attached by one or another to the same facts : 

 these are difficulties inherent in the very nature of our study, 

 while to many minds they increase rather than diminish its 

 attr.actions. Nevertheless the progress of morphology in late 

 decades has plainly been towards a truer appreciation of how 

 divers forms have originated, and so towards a better recognition 

 of aftinities. Seeing that this is clearly the main trend, we 

 may take heart as to the advancement of morphological know- 

 ledge. We .shall not allow ourselves to be deterred by reason 

 of the want of finality or the deficiency of evidence, however 

 strongly we may feel the weight of these difliculiies. We shall 

 rather try to make the Iwst of such evidence as we possess, with 

 the full confidence that, however insoluble the problem of 

 descent in.iy really be, inquiry along scientific lines will at least 

 lead us nearer to the goal. 



