246 GROWTH PRINCIPLES AND THEORY 2 



Rensch (1954) has advanced similar viewpoints, based upon a large amount 

 of factual evidence, and his discussion of evolutionary allometry deserves particular 

 attention. The general conclusions are that no evolutionary factors beside those 

 known today need to be presupposed in macro- (transspecific) evolution. It ought 

 to be considered, however, that the characteristics of organisms essentially are 

 system properties resulting from interactions within the organism. Explanation, in 

 principle, of evolutionary changes of the organism as a whole is possible by 

 taking into account especially the following factors : 



1. Pleiotropic gene action, "constructive" genes acting in the way of harmonious 

 transformations. 



2. Among such constructive genes are especially those influencing body size and 

 hence, by way of allometric growth, the proportions of the body. Any phylo- 

 genetic change of body size may entail complex effects the analysis of which 

 facilitates causal insight into the holistic and harmonious character of phylo- 

 genetic changes. If, according to Cope's rule which is valid in most taxonomic 

 o-roups, a progressive increase in body size takes place in an evolutionary series, 

 these changes in proportion, though only quantitative at first, may eventually 

 lead to changes in the groundplan entailing quaHtatively new characteristics. 

 This is especially evident in eyes of arthropods where image-perception is only 

 possible if a few hundreds of ommatidia are present; the central nervous system 

 where more complex instinctual or learning behavior becomes possible with 

 increased number of neurons; or the teeth which gain new functions with 

 excessive size. 



Quantitative increase of certain parts in consequence of their positive allometry 

 may provide regions which subsequently can be used for new functions. Thus 

 the positive allometric growth of the forebrain in comparison to the brain as a 

 whole makes possible the progressive shift of higher functions to the forebrain 

 as found in the vertebrate series. In particular, the increase of the cortex provides 

 reo^ions which eventually become Broca's motor speech center and hence an 

 essential factor in the evolution of man. 



In extreme cases changes of proportions lead to excessive formations or, con- 

 versely, to nidimentation. This interpretation is confirmed by the fact that ortho- 

 genetic excessive formations almost always appear in giant forms of the respective 

 groups (horns of titanotheres, antlers o^ Megaceros, excessive appendices in beetles, 

 etc.). Histological differentiations and basic physiological functions are altered 

 with changes of morphological proportions showing that hereditary factors 

 are "constructive genes" effectuating transformations of the organism as a 

 v^hole. 



J. A third factor is compensation due to competition for space or building 

 materials. It may result in excessive development of certain organs and becoming 

 rudimentary of others. Phenomena of experimental and phenotypical compen- 

 sation {cf. p. 232) can serve as a model for evolutionary and genotypical ones. 

 Thus, extirpation of an organ can lead to hyperplasia of another; conversely, 

 additional requirements for material in a regenerating part may entail reduction 

 of others. Corresponding phenomena of competition and compensation are found 

 in evolution ; as, e.g. in sabre-toothed cats where extreme increase of the canines 



