September 24, 19 14] 



NATURE 



'05 



the pteridosperms. This unsatisfactory jjosition has 

 now been resolved by Professor Lignier, who has re- 

 cently described, under the name of Mittagia, a fossil 

 from the Lower Westphalian, which bore sori of which 

 the sporangia contained four megaspores, while the 

 outer tissues of the sporangia resembled those of 

 Lagenostoma. Pending the discovery of further 

 specimens, these observations may be welcomed as 

 filling with all probability a conspicuous gap in the 

 evolutionary sequence of known forms. 



From the rapid survey which I have been able to 

 give you of some of the more notable Australasian 

 ferns of relatively archaic type, it is clear that they 

 have a very interesting and direct bearing upon the 

 phylesis of ferns. The basis upon which conclusions 

 as to phyletic sequence are arrived at is at root that 

 of the natural system of classification generally — the 

 recognition, not of one character, or of two, but of as 

 many as possible, which shall collectively serve as 

 criteria of comparison. In the case of the Filicales 

 we may use the characters of :■ — • 

 (i) External form. 



(ii) Constitution, as shown by simple or complex 

 segmentation, 

 (iii) Dermal appendages, hairs or scales, 

 (iv) Stelar structure, simple or complex, 

 (v) Leaf-trace, coherent or divided, 

 (vi) Soral position, 

 (vii) Soral construction, 

 (viii) Indusial protections, 

 (ix) Sporangial structure, and mechanism of de- 

 hiscence, 

 (x) Spore-output. 



(xi) Spore-foim, and character of wall, 

 (xii) Form of prothallus. 



(.xiii) Position of the sexual organs, sunken or super- 

 ficial, 

 (xiv) Number of spermatocytes, and method of de- 

 hiscence. 

 (xv) Embryology. 



In respect of all these criteria progressions of char- 

 acter may be traced as illustrated by known ferns, and 

 probably other criteria may emerge as study pro- 

 gresses. In each case, upon a footing of general com- 

 parison, checked as opportunity offers by reference to 

 the stratigraphical sequence of the fossils, it may be 

 possible to distinguish with some degree of certainty 

 what is relatively primitive from what is relatively 

 advanced. Thus, the protostele is generally admitted 

 to be more primitive than the dictyostele, the simple 

 hair than the flattened scale, and a high spore-output 

 than a low one. 



Applying the conclusions thus arrived at in respect 

 of the several criteria, it becomes possible upon the 

 sum of them to lay out the species and genera of ferns 

 themselves in series, from the primitive to the ad- 

 vanced. In proportion as the progressions on the basis 

 of the several criteria run parallel, we derive increased 

 assurance of the rectitude of the phyletic sequences 

 thus traced, which may finally be clinched, as oppor- 

 tunity offers, by reference to the stratigraphical oc- 

 currence of the corresponding fossils. This is in brief 

 the phyletic method, as it may be applied to ferns. 

 It may with suitable variation be applied to any large 

 group of organisms, though it is seldom that the oppor- 

 tunities for such observation and argument are in any 

 sense commensurate with the requirements. Perhaps 

 there is no group of plants in which the opportunities 

 are at the moment so great as in the Filicales, and 

 they are yielding highly probable results from its 

 application. 



The greatest obstacle to success is found in the 

 prevalence of parallel development in phyla which are I 

 believed to have been of distinct origin. This is 



NO. 2343, VOL. 9Ai 



exemplified very freely in the ferns, and the sys- 

 tematist has frequently been taken in by the resem- 

 blances which result from it. He has grouped the 

 plants which show certain common characters together 

 as members of a single genus. Sir William Hooker 

 in doing this merged many genera of earlier writers. 

 His avowed object was not so much to secure natural 

 affinity in his system as readiness of identification : 

 and consequently in the " Synopsis Filicum " there are 

 nominal genera which are not genera in the phyletic 

 sense at all. For instance, Polypodium and Acro- 

 stichum, as there defined, may be held from a phyletic 

 point of view to be collective groupings of all such 

 ferns as have attained a certain state of development 

 of their sorus ; and that they are not true genera in 

 the sense of being associated by any kinship of 

 descent : this is shown by the collective characters of 

 the plants as a whole. Already at least four different 

 phyletic sources of the Acrostichoid condition have 

 been recognised, and probably the sources of the 

 Polypodioid condition are no fewer. Such "genera*" 

 represent the results of a phyletic drift, which may 

 have affected similarly a plurality of lines of descent. 

 It will be the province of the systematist who aims 

 at a true grouping according to descent to comb out 

 these aggregations of species into their true relation- 

 ships. This is to be done by the use of wider, and 

 it may be quite new, criteria of comparison. Advances 

 are being made in this direction, but we are only as 

 yet at the beginning of the construction of a true 

 phyletic grouping of the Filicales. The more primi- 

 tive lines are becoming clearer : but the difficulty will 

 be greatest with the distal branches of the tree. For 

 these represent essentially the modern forms, they 

 comprise the largest number of apparently similar 

 species, and in them parallel development has been 

 most prevalent. 



If this difficulty be found in such a group as the 

 Filicales, in which the earlier steps are so clearly 

 indicated by the related fossils, what are we to say for 

 the Angiosperms? Our knowledge of their fossil pro- 

 genitors is very fragmentary. But they are repre- 

 sented now by a multitude of forms, showing in most 

 of their features an irritating sameness. For instance, 

 vascular anatomy, that great resource of phyletic study 

 in the more primitive types, has sunk in the' Angio- 

 sperms to something like a dead level of uniformity-. 

 There is little variety found in the contents of embrjo- 

 sacs, in the details of fertilisation, or in embryology. 

 Even the ontogeny as shown in the seedling stages 

 affords little consolation to the seeker after recapitula- 

 tion. On the other hand, within what are clearly 

 natural circles of affinity there is evidence of an extra- 

 ordinary readiness of adaptability in form and struc- 

 ture. Such conditions suggest that we see on one 

 hand the far-reaching results of parallel development, 

 and on the other the effects of great plasticity at the 

 present day, or in relatively recent times. Both of 

 these are points which prevent the ready tracing of 

 phyletic lines. In the absence of trustworthy sugges- 

 tions from palaeontology, the natural consequence is 

 the current state of uncertaint\- as to the phyletic 

 relations of the Angiosperms. 



Various attempts have been or are being made to 

 meet the difficulty. Some, on the basis of the recent 

 observations of Wieland and others, are attempting 

 along more or less definite monophyletic lines to con- 

 struct, rather by forcible deduction than by any 

 scientific method of induction, an evolutionary^ story 

 of the Angiosperms. I do not anticipate that any 

 great measure of success, beyond what is shown in 

 a v'er\- polysyllabic terminology-, and an appearance of 

 knowing more than the facts can quite justify, will 

 attend such efforts. It w-ould seem to me to be more 

 in accord with the dictates of true science to proceed 



