The Source of Variability 



65 



at a greater proportionate rate than the head. Thus at maturity the 

 human head and trunk have different proportions than at an early 

 stage in development (Fig. 24). There is a similar difference in 

 growth rate between the muzzle and cranium of horses (Reeve 

 and Murray, 1942). This differential development in relation to 

 absolute size has been plotted for (1) the ontogenetic develop- 

 ment of the domestic horse, (2) horse breeds of different sizes, 

 and (3) fossil horses ranging from the small ancestral horse Hyra- 

 cotherium to the living, much larger horse, Equus (Fig. 25). In 



45 



41 



37 



33 



29 



26 



23 



20 



16 



16 



14 



12 



10 



9 



8 



7 



6 



5 



Plesihippus simplicidens- 



.4 



-?.^ 



HIppidium neogaeum 



Neohippanon whitneyi 





Pliohippus lullionus 

 ! — Hypohippus osborni 



Merychippus sejunctus 



Mesohippus intermedius 



Mesohippus bairdi 



"Eohippus venticolus 

 'Hyrocothenum 



+ Equus sp 



© Four -toed horses 



O Three -toed horses with 

 low -crowned teeth 



• Extinct genero with 

 high -crowned teeth 



6 7 8 9 10 12 14 16 18 20 23 26 29 

 LENGTH OF CRANIUM (CM.) 



Fig. 25. AUometry in horses. The length of face increases disproportionately 

 as the skull increases in size. A, growth pattern in the modern horse Equus 

 cabaUus; B, growth pattern of slightly different proportions which fits Hijraco- 

 therium and certain other genera. (From Reeve and Murray.) 



