GENERAL CONCLUSIONS 333 



consideration of this scheme, it will be seen that vitaHstic hypotheses 

 such as that of Driesch (1908) are unnecessary. Equally it seems 

 improbable that there can be any simple theory of embryogenesis and 

 morphogenesis. J. T. Bonner (1952) regards morphogenesis, or the 

 emergence of an organismal pattern, as being referable essentially to 

 the progressive, constructive processes of growth, morphogenetic 

 movement and differentiation : and to the limiting of these processes 

 in various ways by intrinsic and extrinsic factors which check, guide and 

 canalise them. He also advances the thesis that a sustaining aim should 

 be a search for a micro-theory which would explain in a satisfying 

 manner the general phenomena of development, such a theory being 

 more likely to afford an insight into the inner mechanism of the relevant 

 phenomena than any other kind of theory. By a micro-theory he does 

 not imply that all the phenomena of biology are to be brought down to 

 physics and chemistry: some kind of intermediate micro-theory may 

 prove quite satisfactory. Bonner does not advance any micro-theory, 

 but he indicates that the kind that would be satisfying would be one in 

 which, as in crystal structure, the organism could be envisaged as being 

 essentially constructed of small repetitive units. Having in mind the 

 complex nexus of factors involved in embryogenesis, it seems improb- 

 able to the present writer that this kind of theory will prove adequate. 

 The diffusion reaction theory of morphogenesis (Turing, 1952; 

 Wardlaw, 1953) could be regarded as contributing to a micro-theory. 

 But, during the individual development the reaction system will change 

 constantly, each successive biochemical pattern being partly deter- 

 mined by, and based on, the preceding ones, and in turn helping to 

 determine those which follow. 



THEORY OF RECAPITULATION 



The theory of recapitulation, or the biogenetic law — that the 

 ontogeny of the individual organism is a recapitulation of the phylogeny 

 of the race — as proposed by Haeckel (1866, 1875), has not been generally 

 accepted by botanists or zoologists, though it retains a certain historical 

 interest. According to this theory, phylogeny determines ontogeny, the 

 development of an individual organism affording a kind of history of 

 its race, the adult states of the ancestors being repeated in a condensed 

 form during the early development of the descendants. There is some 

 evidence which, at first sight, seems to support Haeckel's view; but 

 on closer analysis the theory has not been found valid; in fact, much 

 evidence has been urged against it. Haeckel's theory is closely com- 

 parable with that of Muller (1864) who held that, in its development 

 from the egg, an animal might either pass through and beyond the 

 ontogenetic stages of its ancestor, i.e. eventually overstepping the 



