270 THE SPORE-PRODUCING ORGANS [CH. 



From the description of details and the discussion of them given above it 

 appears that the sporangium of the Filicales provides much material for their 

 comparative treatment. Their sporangia range from structures relatively 

 massive in their several parts and short-stalked, with large spore-output 

 from the individual sporangium, and with non-specialised mechanism for 

 their dispersal, to relatively small structures, more delicate in their several 

 parts, often long-stalked, with small spore-output from the individual spor- 

 angium, and with highly specialised mechanism for their dispersal. The 

 two ends of the series are in strong antithesis to one another, though the 

 intermediate gradations between them show a range of most gentle steps, 

 structural, mechanical, and numerical. There might be some possible doubt 

 of the reality of these steps as marking a true evolutionary sequence were 

 it not for the decisive evidence of Palaeontology. The facts derived from 

 the fossils leave no doubt that the Eusporangiate was the prior type. Scott 

 has remarked {Fossil Botany, 3rd Edn. Vol. i, p. 366), "It is doubtful if the 

 distinction between Eusporangiate and Leptosporangiate Ferns existed in 

 Palaeozoic times — in other words, whether the development of the sporan- 

 gium from a single cell had yet been arrived at.... The conclusions... as to 

 the relative antiquity of the Eusporangiate type are thus amply justified by 

 palaeontological evidence." The facts thus appear to establish the general 

 sequence of forms from the Eusporangiate to the Leptosporangiate, as a 

 valid evolutionary progression. Internal evidence indicates, however, that 

 it was not monophyletic. Similar modifications have appeared in Ferns of 

 divers affinity, as indicated by features other than those of the sporangia 

 themselves. It is upon the sum of the characters rather than upon the 

 sporangial characters alone that any such decisions must rest. When this 

 is grasped, together with the biological probability that underlies the various 

 details of soral and sporangial modification, it appears that there has been 

 a general phyletic drift in the evolution of the Filicales : that it has involved 

 progressions worked out in a number of distinct evolutionary lines : and 

 that these have led from a relatively massive construction, typified in the 

 more primitive forms by the Eusporangiate sporangium, to a relatively 

 delicate and precise construction, typified by the later and derivative 

 Leptosporangiate sporangium. 



Nevertheless the actual spore-output as a whole does not at the same 

 time fall in any marked degree. In some living Ferns, such as the larger 

 Tree Ferns, it is probably as high as in any of their predecessors. The 

 important advantage of high propagative capacity is secured in them by 

 the compensating elaboration of the sorus. In fact the evolution of the 

 sorus and of the sporangium have marched along complementary lines. 

 As the individual sporangium diminished, and its output fell though its 

 mechanism became more precise, the sorus held its own in productive 



