410 INTRODUCTION TO EVOLUTION 



in size of the facial rudiment at a very early stage of growth." In terms of 

 the allometry equation this may have represented an increase in the initial 

 growth constant, b (Simpson, 1953a). We recall that Merychippus initi- 

 ated many adaptations to life on dry plains. Accordingly, the change in b 

 seems to have occurred in connection with the acquisition of hypsodont 

 (continuously growing, high-crowned) molar teeth. From Pliohippus on 

 through the ancestral species of Equus facial growth and cranium growth 

 may well have been nearly isometric as they are in later stages of the 

 ontogeny of Equus (upper portion of line A, Fig. 18.3) but we need more 

 data on this point. 



By way of contrast to the line leading to Equus, the "forest horse," 

 Hypohippus, is included in Fig. 18.3. Note that the point representing it 

 falls at some distance from either of the two lines. Hypohippus did not 

 have hypsodont teeth; its face was only three quarters as long as that of a 

 modern horse having the same cranium length (Reeve and Murray, 1942). 



What has allometry to do with evolution? As noted previously, genes 

 control the rates at which developmental processes, including those of 

 growth, occur in the body. Thus, for example, genes would control the 

 rate at which the horn of our hypothetical rhinoceroslike animal (Fig. 

 18.2) would grow; other genes would control the rate at which the length 

 of the head as a whole would increase. In the example shown we need only 

 suppose that the genes controlling growth of the horn determine that the 

 horn shall grow at a more rapid rate than the rate of growth of the body as 

 a whole. Then, if for any reason the head becomes larger, the horn will 

 automatically become disproportionately larger, as shown in the figure. 



As a result of this phenomenon some of the burden is removed from the 

 back of natural selection. We need no longer ask: Is it important for a 

 large rhinoceros to have a proportionately much longer horn than a small 

 rhinoceros has? Nor need we attempt to imagine how, in evolutionary his- 

 tory, a horn could be increased in length by gradual accumulation of mu- 

 tations each of which increased length by a small amount. Perhaps the only 

 matter of sufficient importance to be acted on by natural selection was the 

 matter of total body size, including head size. Perhaps it was important 

 for the animals to become larger. Judging by the number of evolutionary 

 lines in which increase in size is found, this supposition seems highly proba- 

 ble. If so, natural selection would favor the production of larger and larger 

 animals. In this event, and if the animals possessed a horn growth rate 

 greater than the growth rate of the body as a whole, the disproportionately 

 longer horn would develop automatically as a sort of by-product. Just so 

 long as having a longer horn was not positively detrimental, natural selec- 



