VIII GROWTH AND EVOLUTION 245 



(c) Changes of the organism-as-a-whole as growth problem 



The principle of allometry gives some insight into the most difficuk problems 

 of evolution, those of coadaptation, harmonious transformation of the organism as a whole 

 and orthogenesis — problems that are difficult to reconcile with the conventional 

 doctrine regarding evolution as a chance product of undirected mutations 

 (Bertalanffy, 1942a). Changes in body size lead automatically to changes in 

 proportion by way of allometric growth. If, for example, an increase in body 

 size has selective advantage, this will lead to a transformation of the body as a 

 whole. Such change therefore does not influence one characteristic only but to 

 a greater or lesser degree the entire organism. Furthermore, changes in one part 

 will also have effects upon other parts by way of developmental correlations as is 

 known of "superordinate" genes. For example, the development of the human 

 spine (Fischer, 1931) is controlled by a pair of alleles, the dominant inducing a 

 craniad, and the recessive a caudad shift. Under the influence of the respective 

 genes, in the first case ribs or rib rudiments develop at the last cervical vertebrae, 

 and in the second case, the first thoracic vertebrae do not have ribs. This change 

 does not affect the spine only, but is accompanied by a corresponding shift in the 

 dorsal musculature, the nerve stem with its plexus, the sternum and the position 

 of the diaphragm, etc. In a similar way, allometric changes provide a possible 

 mechanism of harmonious transformation and change of the organism as a whole. 



Furthermore, hereditary changes of growth processes are caused in many cases by 

 hereditary changes of the hormonal balance. These, of course, affect the organism 

 as a whole. For example, many characteristics of man in comparison to the 

 anthropoids can be considered as effects of changed hormonal balance, particu- 

 larly with respect to the hypophysis (Versluys et al., 1939) : the reduction of hair, 

 the gnathic index, the retardation of growth, of sexual maturation and of closure of 

 the sutures of the cranium. Conversely, an ape-like condition can reappear in 

 the case of hormonal dysfunction (hirsutism, acromegalia, prognathism, pubertas 

 praecox, etc.). Sometimes pheno typical changes can be induced by administration 

 of hormones which, if they were genotypical, would represent major evolutionary 

 steps. An example is presented by experiments with Periophthalmus, a fish occasion- 

 ally stalking on land : Prolonged administration of thyroxin leads to adaptations 

 corresponding to those of terrestrial animals, such as cornification of the epidermis, 

 transformation of the pectoral fins into extremities used lever-fashion, etc. (Harms, 



1935)- 



Lastly, cases of evolutionary allometry such as the evolution of the titanotheres, 

 of the brain in mammals, or Megaceros, are paradigms of apparent directedness 

 or orthogenesis in evolution. Such cases show, however, that orthogenesis is not a 

 mysterious "force" or "purposiveness" but follows definite laws. Orthogenetic 

 enlargement of organs may eventually lead to excessive formations and "blind 

 alleys" in evolution as was probably the case in the titanotheres and in Megaceros. 

 It may also lead to differentiations which are at first not required in the way of 

 adaptation, but may be employed later on for higher performances. This is 

 probably the case in the evolution of the human brain, primitive races having the 

 same degree of cephalization as man in high cultvires but apparently not making 

 complete use of it (Versluys et al., 1939). 



Literature p. 253 



