CONCLUSIONS 



elusion that the evolutionary potential of the Pteridophyta as a source of major 

 innovations into the earth's flora is running down. 



The reasons for this seem very varied even on the flimsy evidence we have. High 

 specialization is known to check evolutionary potential in other groups, and this is per- 

 haps the condition in Isoetes and Hymenophyllum. Selaginella appears to be stereotyped, 

 perhaps from structural reasons, for among the several hundred species which differ in 

 details, none is sufficiently distinct to tempt taxonomists to call it a genus. Selaginella 

 now seems incapable of any form of macroevolution and has probably been in this con- 

 dition for millions of years, even if it was not always so. 



That cytological conditions in themselves can be factors which lead to a slowing down 

 of evolutionary activity is, however, a conclusion which more particularly emerges from 

 the present study and which, if true, is perhaps one of the more valuable parts of it. In 

 the Osmundaceae we have already remarked upon the coincidence of a primitive 

 morphology and extreme uniformity of nuclear structure among the modern genera. 

 The cytological condition here seems to be primitive, and it is perhaps also relevant to 

 recall the marked difficulty recorded in Chapter 3 of obtaining viable sporophytes with 

 any considerable departure from the normal complement of chromosomes. Another 

 observation, well known to gardeners, is the great scarcity of mutant forms of even much- 

 used ornamental plants like Osmunda regalis in contrast to the profusion of monstrosities 

 which most other British species have yielded. We seem here to be dealing with an 

 ancient group in which the nuclear constitution has, for unknown reasons, become so 

 stable that it is now almost incapable of change either genetically or cytologically. To 

 this fact the long retention of primitive morphological characters may perhaps be due, 

 and the success of such species as we have, over such very prolonged periods of time, may 

 perhaps be attributed more to the physiological resilience which makes them so 

 tenacious of hfe under very varied conditions of climate or culture, than to any 

 specially adaptive features in their morphological characters as such. 



That high chromosome number in itself may act as an internal factor tending to 

 slow down evolution is strongly suggested by a comparison between the two almost 

 equally ancient genera oi^ Selaginella and Equisetum. In the one, Selaginella, we have 800 

 taxonomic species. In the other, Equisetum, a genus which in temperate latitudes is at 

 least as successful if area of ground occupied is a valid guide, there are only some two 

 dozen species on the whole earth's surface. Both have existed for milUons of years, and 

 should have had ample time to develop their full potentiahties. Is the difference be- 

 tween them perhaps in part due to the fact that genetical innovations become effective 

 more easily where the haploid chromosome number is 9 than when the nucleus has be- 

 come loaded up as it were, by aneuploidy or by polyploidy, to a prevailing gametic 

 number of 108? If this is so, we may include high chromosome number among the factors 

 which lead to evolutionary stagnation, and see in it perhaps one of the commonest 

 reasons why so many of these ancient groups have remained primitive in spite of a 

 tenacity to life which has ensured their survival. We may also predict that the Fihcales, 

 that large and lively group from which so much of our evidence has been derived, are 

 tending that way and are almost certain, in the future, to become engulfed in their 

 own increasing complexity and probably to succumb, except for their hardiest or 



Mpc 289 ^9 



