CONCLUSIONS 



the persistence of low chromosome numbers, and they only serve to enhance the 

 generality of the rule that where polyploidy is extensively practised, the low-numbered 

 species are the most likely to die out first. This, as we have seen, may mean that in a 

 really ancient group such as Equisetum, the Psilotales or Ophioglossum, the origin of a 

 series may become effaced, and it can then only be reconstructed in imagination from 

 indirect evidence. 



An unequivocal reason for this behaviour is not easy to diagnose, though suggestions 

 about it can of course be made. The mutational character of allopolyploid species, 

 i.e. their sudden appearance, may have something to do with it. This may perhaps be 

 specially important if, as must often have happened, the two parent species have been 

 brought into juxtaposition by some unusual circumstance such as changing climate, 

 which may destroy old habitats together with their inhabitants and liberate new ecologi- 

 cal sites for colonization, provided only that suitable colonists, not too closely bound to 

 the older conditions, can step in while the opportunity is open. Under these circum- 

 stances, the possible advantages of mutational change (saltation in the sense of the old 

 mutation theory), as opposed to the slower process of adjustment to changing conditions 

 by means of natural selection acting on the raw materials (e.g. polygenes, major genes 

 or recombinations of biotypes) provided by genie mutations, are obvious, and the effect 

 may well be decisive in determining which forms can survive. Another possible 

 explanation for the replacement of low- by high-numbered species is hybrid vigour in 

 the latter; but this is almost certainly to be discounted in view of the immense lapses 

 of time which must be involved in the changes which we are considering. 



That to a limited extent diminution of chromosome number can also occur is suggested 

 in the Pteridophyta by the solitary evidence o{ Hymenophyllum (Chapter i6) and perhaps 

 Doodia (Chapter 12). With fuller knowledge other and clearer examples would almost 

 certainly be found. These would, however, in no way disturb the generality of the 

 rule that in this group the aneuploid changes no less than the polyploid ones, though 

 at a slower rate, tend on balance to inerease chromosome numbers if a long enough 

 period of time is considered. This observation could not have been made on the Cruci- 

 ferae alone, and it is indeed possible that the Flowering Plants may respond to old age 

 differently. If they do not, or if this phenomenon is at all widespread, we must 

 recognize it as one at least of the possibly numerous factors which lead ultimately to 

 th^ decay of once dominant groups. 



To pursue this idea further, we may survey the Pteridophyta and ask ourselves where, 

 if anywhere, could a new great group of the future arise ; but we should be at a loss for 

 an answer. If we have diagnosed the evidence aright, that all great groups must start 

 from simple beginnings with low chromosome numbers, the choice is limited. We have 

 Selaginella, Isoetes hystrix, Hymenophyllum and Osmunda. Only an irrepressible optimist 

 would, however, expect big developments now from such peculiar and apparently 

 stereotyped and specialized forms as the first three, and Osmunda, though probably still 

 primitive, is known to have had its biggest burst of macroevolution in the very distant 

 past, and all the fossil evidence we possess gives no instance of a recrudescence of effective 

 evolutionary activity after so great a lapse of time except perhaps of the 'gerontic' sort, 

 i.e. as a senile outburst preceding extinction. We seem forced therefore to the con- 



288 



