284 SUMMARY OF RESULTS [ch. xlix 



is found in those steadily progressive reductions of mass, of complexity 

 in cell-structure, and of spore-output which mark the passage from the 

 Eusporangiate to the advanced Leptosporangiate state. These were not 

 executed in one phylum only, but independently in many. There has been, 

 in fact, a great Class-Progression, carried out by homoplastic steps, originated 

 and executed independently in the several phyla of Ferns. Collectively 

 those independent steps constitute a general progressive drift which presents 

 a very complicated problem to the evolutionist. They raise the question of 

 causality in a peculiarly complex form. In particular we may ask. Were the 

 causal stimuli that brought it all about external or internal : or is it not 

 possible that both may have interacted to produce the results which we see? 

 Such questions arising out of the phyletic study of Ferns cannot be held as 

 applying to them alone. They suggest that similar progressive phenomena 

 involving wide homoplasy will emerge in other Classes of organisms when 

 subjected to similar phyletic study. 



