CONCLUSIONS 



luckiest descendants, as the inevitable consequence of the working of their internal 

 evolutionary machinery. 



This is a highly mechanistic conception which disregards the actual morphology of the 

 plants themselves and also their degree of structural 'adaptation' to their mode of life, 

 not from any prejudice against the Darwinian or other theories of evolution but by the 

 force of logic in the facts before us. Adaptation there must clearly be or a plant cannot 

 survive, and if an ecological niche suitable for such survival is not available at the right 

 time a potential new species will not become established or an old one will die out. But 

 to look here for the mainspring of Macroevolution seems to me personally a fruitless 

 quest. That once established, most species have a very considerable power of physio- 

 logical adjustment to their environment by Microevolution has been proved in the 

 Flowering Plants by the work on experimental ecology initiated by Turesson (1922) 

 and powerfully amplified among others by Clausen, Keck and Hiesey (i 940-1 948) in 

 their admirable Experimental Studies in the Mature of Species. Such work has not yet been 

 extended to the Pteridophyta and it is very desirable that it should be. It shows 

 unmistakably that it is possible and indeed probable that th^ pace of evolution may be 

 accelerated or retarded by environmental pressure, since the greater the opportunity 

 the more frequently will new forms become established, and, conversely, the more 

 extreme the changes in a given environment the greater will be the number of species 

 to die out. Great evolutionary activity may therefore accompany or follow periods of 

 violent climatic oscillations, such as an ice age. But that the major evolutionary trends 

 have been primarily caused by such changes seems impossible to believe. We have been 

 studying, in the Pteridophyta, an ancient group which has survived innumerable 

 cosmic vicissitudes and which had already become subdivided almost at the dawn of 

 the fossil record into the various main branches which still survive, together with others 

 which have died out. In our study of species we have encountered several examples of 

 parallel evolution and a great many examples of the production of new forms by 

 mechanical processes such as hybridization, polyploidy and so on. At the same time, 

 in studying the major groups as a whole, we have in the last few paragraphs had 

 repeatedly to call attention to a variety of different mechanisms by means of which 

 evolution seems to have been slowed down or stopped. At no point have we been con- 

 strained to look outside the organism for a directive influence. 



This is perhaps the one point at which serious comment on generalized evolutionary 

 theories may perhaps be offered, while observing the limitations on the scope of the 

 argument explained in earlier pages. In most general theories, from the time of Lamarck 

 almost to the present day, attention has primarily been directed to the search for some 

 mechanism either outside or inside a living plant or animal by means of which, in the 

 course of generations, its character, appearance and functions might become changed 

 in an orderly manner, and without which these might be presumed to remain unchanged. 

 We may now perhaps ask ourselves whether an avoidable difficulty may not have been 

 introduced into the quest by this last assumption, unconsciously accepted as it usually is. 

 The emphasis which we have repeatedly had to lay on the detection of mechanisms by 

 means of which evolution appears to have been stopped, may suggest that a more help- 

 ful basic assumption will perhaps be found to lie in the truism that evolution, as such, is 



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