CONCLUSIONS 



a phenomenon for which no cause need be assigned other than the fundamental 

 instabihty of hving matter. 



The apparent orderhness of evolutionary progressions, when viewed from a distance 

 against the background of the geological time scale, may perhaps only express the fact 

 that at any one period, in a given organism with a given structure and development, only 

 a limited number of types of innovation are possible without prejudice to its efficiency 

 as an individual, but that such changes as can occur most easily will do so repeatedly. 

 The majority even of potentially successful innovations will be expected to disappear 

 without trace, together with the vastly greater number of intrinsically unsuccessful ones, 

 since the accident of opportunity must also coincide. Given long enough, however, an 

 inherent instability in certain directions rather than in others will express itself taxo- 

 nomically, as parallel evolution or an orthogenetic* trend. Both of these have been 

 encountered in the Pteridophyta. We need only recall the soral characters o{ Dryopteris 

 or Athyrium and the complex apparatus of obligate apogamy for examples to demon- 

 strate the repeated origin of similar innovations in different places. Even the polyphyletic 

 origin of a species is perfectly possible in certain cases, e.g. hexaploid Polypodium. On 

 the other hand, the polyploid series itself which has been met with so abundantly could 

 be thought of as an example of the stepwise increase of a character caused by the 

 repeated introduction of similar innovations in time, which could easily have a 

 morphological equivalent. If indeed, to make this suggestion more precise, we replace, 

 in imagination, apogamy by heterospory and advancing polyploidy by progressive 

 precocity of the gametophyte, we are almost within sight of macroevolution leading 

 to the seed habit, without seriously overstraining creduhty as to what might reasonably 

 be supposed to have happened. 



That the polyploid series is a valid analogy, indeed perhaps even a special case, of 

 an orthogenetic trend, is further suggested by its apparent relevance to another observa- 

 tion, no less remarkable for being famihar, namely, the surprising tendency of biological 

 systems to change in the direction of increasing complexity. We are told by physicists 

 that one of the most fundamental statements of experience in the inorganic world is the 

 second law of thermodynamics which says that entropy, by which is meant randomness 

 or disorder, always tends to increase. Yet here before us in both plant and animal 

 kingdoms we find the most elaborate kinds of order spontaneously generating them- 

 selves, not indeed out of nothing, but from simpler beginnings, and this, even at the 

 price of ultimate extinction from over-elaboration. What is the reason for this, at first 

 sight, flagrant contradiction? That the energy of respiration is sufficient answer is 

 difficult to believe. 



This may perhaps suggest that to understand evolution in general terms we need to 



* Although the sense in which this word is used here will probably be clear from the context and from 

 what follows above, it should perhaps be emphasized that it does not imply the existence of any ' inscrutable 

 creative force' as is sometimes assumed (e.g. Sewall Wright, 1949). A useful definition may be quoted 

 from the glossary to the symposium on Genetics, Palaeontology and Evolution (Jepsen, Simpson & Mayr, 

 1949) which has reached me at the last moment before this manuscript goes to press and which cannot 

 unfortunately be discussed as a book, '...orthogenesis. Evolution continuously in a single direction over a 

 considerable length of time. Usage differs considerably, but the term usually carries the implication that the direction is 

 determined by some factor internal to the organism — ' 



291 ^92 



