CHAPTER XVII 



ORGANOGRAPHIC COMPARISON OF THE FILICALES 

 WITH OTHER PLANTS 



In the foregoing Chapters an analysis has been made of the several out- 

 standing features, somatic and propagative, which may be used as a basis for 

 the seriation of Ferns according to the probable history of their evolution. 

 In each of them, partly on comparison of living forms, partly by reference to 

 the fossil history, the characteristics held to be relatively primitive have been 

 distinguished from those held to be relatively recent in Descent. In these 

 determinations each criterion of comparison has been regarded as standing 

 upon its own footing, and has been judged as far as possible independently 

 of others. But naturally if the conclusions arrived at are sound, it may be 

 expected that the sequences in respect of the various criteria of comparison 

 will run parallel ; and in the measure in which they do so they will gain 

 mutual support. By collecting the conclusions from comparison in respect 

 of the different criteria, and putting them together, it will be possible to 

 construct a composite picture of an archetype which shall embody all the 

 features which are recognised as the most primitive for Ferns. The visuali- 

 sation of such an archetype would thus be based upon actual knoivlcdge of 

 Ferns living or fossil. It can then be compared with the earliest fossil land- 

 plants of which we have detailed knowledge. It is believed that this method 

 is more trustworthy and scientific than any general comparisons of land- 

 plants with Algae, and will be more helpful than these in elucidating the 

 probable evolution of the Class. The reason for holding this opinion is that 

 a comparison of Archegoniate with Archegoniate is nearer than that of an 

 Archegoniate with any Alga, and it is more reliable because it leaves less 

 scope for the slack joint of homoplasy. 



In Chapter III the simple shoot composed of an axis and an acropetal 

 succession of leaves has been recognised as the unit of construction of the 

 sporophyte for the Ferns at large. In the primitive condition the shoot as 

 a whole is iinhranched, as it is at the start of every ontogeny of living Ferns. 

 This condition is often maintained in the adult of the Ophioglossaceae, Os- 

 mundaceae, and Marattiaceae, in which the axis is as a rule upright, a position 

 held to be primitive. Such shoots are radially constructed. The prone position 

 is probably derivative, though it certainly was acquired early, and it follows 

 naturally from the heavy leafage, and the prevailing absence of secondary 

 thickening of the stem. The symmetry of the prone shoots is dorsiventral, 



