HETEROGONY AND EVOLUTION 219 



probable, then the results follow without any need for invok- 

 ing orthogenesis. Natural selection would account for the 

 increase of absolute size, and increase of absolute size would 

 evoke the latent potentialities of the horn's growth-mechanism 

 in the history of the race just as surely as it does, for instance, 

 in the individual deer to-day, or as it undoubtedly did in the 

 individual Titanothere in the Oligocene. No alteration in the 

 hereditary basis for horn-growth need be involved at all, 

 whether determinate or otherwise : the visible horn-changes 

 are automatic results of the effect of size-changes in the un- 

 altered mechanism for horn-growth. 1 



The only remarkable thing about this case is that the 

 mechanism for horn-growth must have been represented in 

 the germ-plasm before it could ever operate — before any single 

 Titanothere was large enough to have a horn at all. And 

 this is undoubtedly surprising. Once more, it makes it ex- 

 tremely difficult to assign any adaptive value to the horns, 

 at any rate in the early stages of their evolutionary develop- 

 ment, when they were represented only by the merest incipient 

 knobs, and still more when they merely existed potentially ! 2 



To take a case nearer home, Parsons (1927) maintains that 

 the British race is showing evolutionary change, because the 

 mean proportions of British skulls have slightly changed 



1 Doubtless some quantitative modifications in growth-ratio of horn 

 took place, as evidenced by the difference in relative horn-size in the 

 various groups ; but this does not affect the main argument. 



Champy (1924, p. 155) takes an almost identical view. He writes : 

 ' Le phenomene d'orthogenese ou rectigradation, lorsqu'il accompagne 

 une augmentation de taille, n'est pas un phenomene evolutif, c'est 

 un phenomene purement physiologique d'equilibre nutritif. La seule 

 evolution qu'il y ait a expliquer est l'augmentation de taille.' 



2 The Asiatic Titanothere genus Embolotherium is distinguished not 

 only by possessing a very large and peculiarly-shaped horn, but by 

 this being formed entirely from the nasal bones, instead of from both 

 nasals and frontals, as in all other genera. During both the ontogeny 

 and the phylogeny of other late Titanotheres, there is a tendency for 

 the horns to grow in an anterior direction as well as to increase in size. 

 It may be suggested that the condition in Embolotherium represents 

 a further step in this evolution — the actual shifting of the growth- 

 centre from the fronto-nasal to the nasal region. This would be a 

 shift of a whole subsidiary growth-region with a gradient of its own, 

 along a main gradient. Somewhat similar shifts of quantitative growth- 

 centres within a single growth-gradient have been already recorded. 

 The most relevant case is the shift of the growth-centre of the female 

 abdomen of Pinnotheres to the terminal segment in place of the sub- 

 terminal segment where it is situated in most Brachyura (p. 95) ; cf. 

 also p. 117 (Copepod body-regions). 



