GENETIC CHANGE AND EVOLUTIONARY CHANGE 413 



easy prey to predators as do small ones. Thus natural selection would tend 

 to favor genetic changes (mutations) making for increase in size. As the 

 horses became larger their faces became longer, not because of any 

 genetic changes, but because genes controlling rate of facial growth deter- 

 mined greater rate of growth than that characterizing the body as a 

 whole. Eventually, at the Merychippus stage, hypsodont teeth evolved in 

 connection with change in diet. This change in tooth structure necessitated 

 changes in the facial skeleton. At that time genetic changes occurred, but 

 since then there has evidently been little further change in the relative 

 lengths of face and cranium. 



Similarly, in most horse lines the three toes retained about the same 

 proportions to each other. But in the Hne leading to the one-toed horses a 

 sudden genetic change occurred, with the resuU that there was an abrupt 

 decrease in relative size of the lateral digits. Presumably this change was 

 favored by natural selection in connection with the development of efficient 

 foot structure adapted for rapid movement on dry plains (see pp. 197-198). 



Thus we see that allometry removes the necessity for postulating large 

 numbers of more or less independent mutations, each affecting some par- 

 ticular structure of the body and each acted on by natural selection. In the 

 evolution of the face and legs of the horse two principal genetic changes 

 seem to have occurred; (1) increase in growth rate of the face, inaugu- 

 rated at the Merychippus stage; (2) decrease in growth rate of the lateral 

 digits when the horses reached a functionally one-toed state. Once inaugu- 

 rated, these changed growth rates persisted. 



So far we have emphasized cases in which the growth rate of a part has 

 been greater than that of the body as a whole. This is called positive al- 

 lometry and is exemphfied in Fig. 18.2. In positive allometry the constant 

 k is greater than 1 . If the value of A' is 1 , growth of the part proceeds at the 

 same rate as does growth of the whole. This is called isometric growth. If 

 the growth of the horn in our rhinoceroslike animal had been isometric, 

 when the head increased in length fourfold (Fig. 18.2E), the horn would 

 also have increased in length fourfold, i.e., would have been 10 cm. long. If 

 the value of k is less than 1, i.e., is a fraction, the part in question increases 

 in size more slowly than does the body as a whole. This process is called 

 negative allometry. If this had been true of our rhinoceroslike animal, the 

 horn of the largest animal (Fig. 18.2E) would have been even less than 10 

 cm. long; it would have been relatively shorter than it was in the smallest 

 individual (Fig. 18.2A). Negative allometric growth may help to account 

 for the relative reduction in size observed in some organs during evolu- 



