APOGAMOUS FERNS. EVOLUTION OF THE SEPARATE SPECIES 



possibility must, however, be recognized. The wide geographical range and apparently 

 relict status of P. cretica in Europe must 

 denote a high antiquity for that species 

 during which time chromosome changes 

 (e.g. translocations and the like) may have 

 taken place, and in the absence of natural 

 selection their effects may have accumu- 

 lated. It is conceivable that all the 

 irregularity of pairing observed both in 

 production of multivalents and of univa- 

 lents may actually have arisen in this way 

 after the apogamous habit was established. 

 This would mean, if true, that for this one 

 species the possibility exists that apogamy 

 itself could perhaps have arisen in a 

 formerly sexual species, not by an act of 

 hybridization, but by a process of internal 

 differentiation of a genetical kind. This 

 explanation is not the most likely one, 

 for it would be expected that traces of a 

 sexual form of similar morphology would 

 be found if mere mutation had produced 

 the apogamy, and there is so far no in- 

 dication that such a sexual form exists. 

 It is, nevertheless, perhaps of importance 

 to raise this alternative explicitly at this 

 point, for, as will shortly be seen, it is an 

 explanation which on cytological grounds 

 is virtually excluded for every other apo- 

 gamous species analysed. 



It may now be suitable to complete the 

 account o{ Cyrtomium. As already explained 

 (Chapter lo) there are now three species 

 instead of one to consider owing to the 

 splitting of the old aggregate ' Aspidium fal- 

 catum' into three microspecies now known 

 as Cyrtomium falcatum Presl, C. Fortunei 

 J.Sm. and C. caryotideum Presl. The first 

 has already been illustrated in Fig. 163, 

 the second is represented in Fig. 180 and 

 the third in Fig. 1 79. All three have been 



Fig. 179. Cyrtomium caryotideum {y^a\\.)Vvc%\ 

 from Uganda. A young, live frond in 

 cultivation. Natural size. 

 Mpc j"yY 12 



