VIII 



GROWTH AND EVOLUTION 



243 



adult organism, organs in different species will grow at different rates; that is, 

 intraspecific allometry constants of organs of different species will, in general, 

 be different from the interspecific constants of these organs. Such relation is 

 apparent in the brain. As ganglion cells do not multiply in post-embryonic 

 development, the relative size of the brain is high at birth. In adult organisms, 

 proportionality to the 2/3-power of body weight is functionally recjuired; hence 

 ontogenetic allometry of the brain is much more negative than interspecific 

 allometry (approximately 0.2 as against 0.66). 



Fig. 44. Changes in the structure of the mesogastric epithelium with increasing body size in 



insects. Note the height of the epithelium and increasing folding, a Ctenophora, b Theobaldia, 



c Lycoria, d Contarinia (gnats). After Partmann, 1948. 



On the basis of allometric relationships numerous rules governing changes 

 of proportions with increasing body size can be stated (Rensch, 1948b, 1954). 

 Larger species have relatively shorter ears, tails, legs, wings and beaks (Allen's 

 rule); relatively smaller brains, eyes (Haller's rule), hearts, livers, kidneys, lungs, 

 thyroids, adrenals, pancreas and hypophyses; a relatively longer gut (this in 

 contrast to the negative ontogenetic allometry of the intestine) ; relatively heavier 

 bones, etc. The rules of changes in proportion going along with evolutionary 

 changes of body size are known sufficiently well to allow for prediction, with a 

 70-80 per cent probability, as to what properties an unknown species of a certain 

 body size (say a rat in the size of a beaver) would exhibit (Rensch, 1954). 



A further important consequence is in the fact that allometric increase remains 

 not confined to quantitative changes; it further entails histological and cytological 

 differentiations (Rensch, 1948a). This was investigated by Rensch and co-workers 

 in numerous organs and species: brain of rodents (C. Schulz, 195 1), of lizards 



Literature p. 253 



