GENERAL CONCLUSIONS 325 



not unlike the fossil Rhyiiia. Here, too, it has been suggested that if the 

 more complex vascular plants were not derived from a Psilopsid 

 ancestral type, they probably passed through that level of organisation 

 in the course of their evolution. 



Although wide phyletic gaps admittedly separate algae, bryophytes, 

 pteridophytes, gymnosperms and angiosperms, the fossil evidence is, 

 nevertheless, generally consistent with the view that these groups 

 appeared in that sequence during the process of evolution. The 

 existence of some kind of linking relationship is thus a reasonable 

 inference. In the present context we may ask if the embryological data 

 either enable us to bridge the phyletic gaps or to apprehend more 

 adequately their nature. 



In his Theory of Transformation, D'Arcy Thompson (1917, 1942) 

 has shown that in a related group of organisms it is possible to indicate 

 homologies, or identities, which were not evident before, by the use of 

 deformed Cartesian coordinates. By this method, morphological 

 differences in organisms of the same affinity can be analysed and, in 

 many instances, can be shown to be more or less simple deformations 

 of an original type. In the view of Richards and Riley (1937) the true 

 field for the use of D'Arcy Thompson's Method of Transformation lies 

 in development and not in evolution, or, as Medawar (1945) says, 'in 

 the process of transforming, and not in the fait accompli.'' The real 

 transformations that underlie the evolutionary transformations are 

 zygotic transformations and these do not lend themselves to treatment 

 by this method. (For recent discussion of this aspect, see Medawar, 

 1945; Richards and Riley, 1937; Richards and Kavanagh, 1943, 1945; 

 Needham, 1942; Woodger, 1945; Wardlaw, 1952.) Woodger has 

 pointed out that although the 'structural plans' (Bauplans) of the adults 

 of two species may be very different, the two may have a Bauplan in 

 common at the unicellular zygote stage; and if we had an adequate 

 theory of cell-organisation we might be surprised to find how little the 

 Bauplan determining the zygote of one species differs from that of 

 another. 



The green algae may be regarded as organisms with constitutional 

 limitations on their capacity for growth to any considerable size and 

 they have remained at a low level of organisation — the simple or 

 branched filament. Nitella, Chara, and some members of the Chaeto- 

 phorales, may be indicated as green algae in which a considerably 

 higher level of organisation has been attained; but they still remain 

 limited in their size development and differentiation. Indeed, it seems 

 reasonable to accept the view of Fritsch (1945) that those green algae 

 which were capable of developing to larger size, and of becoming more 

 highly differentiated, became the progressive and successful plants of 



