VIII 



GROWTH AND EVOLUTION 



247 



goes along with lack of the first premolars, or in horses where increase of the 

 third toe is accompanied by reduction of the second and forth. 



Hence, harmonious and holistic transformations in evolution can be explained 

 by factors such as pleiotropic gene action, changes in proportion by allometric 

 growth and its complex effects, orthoselection in changes of body size, compen- 

 sation owing to competition for building materials, and constructive strains 

 appearing when an upper or lower size limit is approached. There is no need to 

 hypostatize unknown autonomous evolutionary forces, such as a "tendency 

 toward perfection" or the like but, on the other hand, there are laws of organi- 

 zation often manifest as "evolutionary constraint". As opposed both to a vitalistic 

 "autogenesis" and "ectogenesis" considering evolution as being determined solely 

 from outside by chance factors in the environment during geological history, 

 Rensch aptly speaks of bionomogenesis, i.e. evolution as the result of complex 

 causal relationships of the environment as well as in the organism itself. 



100 



80 



60 



40 



20 



Time 



Fig. 46. The uniqueness of the human growth curve. After Bertalanffy, 1951a. 



(d) The uniqueness of man as a problem of growth 



The unique characteristics of man can be envisaged, to a large extent, as a 

 problem of growth, many peculiarities found only in man being consequences 

 of growth phenomena both in ontogenesis and evolution. 



This applies, first, to the human growth curve. The general shape of the growth 

 curve being essentially the same within vertebrates, growth curves of different 

 species can be plotted on the same normal curve, if the scales for time and body 

 size are appropriately chosen (Fig. 46). Even the growth cycles found in mammals 

 (p. Qigff.) do not essentially change the shape of the growth curve. Only one 

 organism is exceptional, namely, man. 



The second part of the human growth curve, beginning at puberty, follows 

 the general pattern. The first part, however, is different. Growth is protracted 

 in infancy and childhood, a new growth cycle being added as it were, to the 

 typical curve. Hence, an "adolescent spurt" (Tanner, 1955), i.e. a second rise of 



Literalure p. 253 



