VIII 



GROWTH AND EVOLUTION 



241 





Eohippus Mesohippus Merychippus Domestic 

 Horse 



0.1 cm 



1 10 



Length of cannon bone 



Fig. 43. Evolution of the equine foot, a In the tridactyl series from Eohippus to Merychippus, 

 the lateral digits become progressively removed from the ground and functionless (O — » A) ; 

 this is followed by their abrupt rudimentation (B —* C). In the ontogeny of the modern 

 horse, the phylogenetic way — > /I is eliminated, b Allometric plot. The line T^ represents 

 the evolution of tridactyl horses (n Hyracotherium and Eohippus, A Alesohippus and Meri- 

 chippus, P a modern polydactyl horse; the line also includes L^, the extinct horse-like camel, 

 Lithopterna) . T' represents development and evolution in unidactyl horses (o fetal recent, 

 • adult recent, + extinct unidactyl horses). L^ is a parallel mutation in a unidactyl 



Lithopterna. After Robb, 1937. 



The case is different in the equine foot (Fig. 43). Here the evolutionary trend is toward 

 elongation of the third toe, while the second and fourth become functionless and rudi- 

 mentary. Within the tridactyl series, simple allometry and continuous transformation are 

 found, as with increasing body size the third toe increases, the second and fourth toes are 

 progressively reduced. In contrast, the transition from tridactyl to unidactyl horses is a 

 discontinuous jump, indicating a deviation, a new turn taken in evolution. Consequently, 

 there is no recapitulation in the ontogenesis of the foot of the modern horse which parallels 

 only the unidactyl but not the tridactyl ancestral series. 



These are examples of evolutionary allometry, i.e. of the comparison within 

 well-established ancestral series whose number is naturally small. Further insight 

 can, however, be obtained by including interspecific allometry, i.e. comparison of 

 related recent species of different sizes which do not represent evolutionary series. 

 To a certain extent, it may be assumed that similar relationships as found in such 

 interspecific comparison also apply for true evolutionary series. 



Such comparison shows, first, that ontogenetic and evolutionary allometry 

 frequently do not coincide. In quantitative expression, this means that intra- and 

 interspecific allometry constants are not identical (Table 16). A rat is not simply 

 an allometrically enlarged mouse (Rensch, 1954). 



The explanation can be sought in the fact (Bertalanffy and Pirozynski, 1952) 

 that the adult organism requires a certain balance of its organs (p. 229f ) for proper 

 functioning, the size-dependence of the organs being expressed by the interspecific 

 constant. 



Lileralure p. 253 



