SCOPE AND OUTLOOK 3 



In the post-Darwinian pliyletic period — comprising the latter part 

 of the Nineteenth Century and the beginning of the Twentieth Century 

 — many botanists were confident that the genealogical relationships of 

 all classes of plants could be truly represented by a unified and coherent 

 taxonomic system; in short, by a monophyletic classification. A 

 widening knowledge of plant form and structure, however, and a 

 growing appreciation of the prevalence of parallel and convergent 

 evolution, brought the realisation that the evolution of plants might 

 perhaps not be adequately represented by a monophyletic genealogy. 

 Indeed, it seemed that the evolution of the several classes or sub- 

 divisions would be more correctly indicated by a system of separate 

 phyletic lines, possibly originating in ancient algal forms. The more 

 comparative morphologists studied this theme, the more they became 

 aware of the difficulty of finding common ancestors for any of the 

 major groups of plants. In fact, gaps occurred at almost every 

 critical point in the record of the evolutionary process ; and this was 

 true not only of the major monophyletic system, but also within 

 individual phyletic lines. So impressed was D'Arcy Thompson (1917, 

 1942) by the prevalence of these 'phyletic gaps' that he remarked that 

 it almost seemed as if a 'principle of discontinuity' was involved. Here 

 it should be said that although the details of plant embryology were 

 certainly not neglected by the evolutionist, many of the conclusions 

 relating to phylogeny were necessarily based on comparisons of the 

 form and structure of adult organisms, especially in the fossils; and it 

 was in these comparisons that the prevalence of phyletic gaps became 

 so evident. Woodger (1945, 1948), however, has indicated that these 

 data may be viewed in another way and that there is great scope for 

 renewed investigations in comparative embryology, provided a new set 

 of concepts can be devised. In his view, organisms which are rather 

 unlike in the adult state may nevertheless have much in common in 

 their early development; and a more adequate study of the factors 

 which are at work in the embryogeny may eventually show that some 

 of the phyletic gaps, which had proved so puzzling to the early investi- 

 gators, may in reality be considerably less wide than had been thought, 

 or they may even be non-existent. The contemporary botanist may 

 thus feel encouraged to go as deeply as possible into the study of 

 embryogenesis and he should constantly search for new ideas and new 

 techniques. Zoologists have made much greater progress than botanists 

 in their studies of embryos by experimental and other means. Here 

 reference may be made to the remarkably ingenious manipulative 

 investigations of Spemann (1936, 1938), Dalcq (1938, 1941) and others, 

 and to studies of biochemical relationships by Needham (1931, 1942) 

 and Brachet (1950). It is not suggested that botanical investigators 



