MEANS AND METHODS OF EVOLUTIONARY CHANGE 361 



crement in length of leg of an ancestral horse, for example. It is true that 

 small effects characterize most well-known mutations, but evidence is ac- 

 cumulating that there are other mutations which have more far-reaching 

 effects. Thus, there seem to be genes controlling the rates at which vari- 

 ous parts of the body increase in size. If a mutation occurs in one of these 

 genes the relative size of the part of the body affected may be greatly al- 

 tered as a result of that single mutation. Suppose, for example, that at a 

 certain point in the evolution of the horse a mutation occurred in a gene 

 controlling the rate at which the legs increased in length as the body in- 

 creased in size. Such a mutation might determine, perhaps, that as the 

 body doubled in size the length of the legs would increase two and a half 

 times (instead of merely doubling as they had formerly done). Since, as 

 we know, the body did increase in size during horse evolution, such a mu- 

 tation would explain the fact that the size increase was accompanied by a 

 disproportionate lengthening of the legs — such a lengthening as was ob- 

 served to occur. 



According to this idea the cumulative action of many little mutations, 

 each adding its increment to length of leg, can be replaced by a single 

 mutation altering the rate at which the leg increases in length as the body 

 increases in size. Such unequal growth of one part of the body relative 

 to another is called allometric growth. Allometry (heterogony) presents 

 possibilities for explanation of some types of evolutionary change with 

 economy in number of mutations postulated. Mathematical formulation 

 of the principles involved, additional examples, and further discussion of 

 the application of allometry to horse evolution will be found in Chapter 18. 



In this connection we may mention that one investigator, Goldschmidt 

 (1940) dissented completely from the idea that "little" mutations can ever 

 be accumulated sufficiently to account for major evolutionary change. He 

 divided evolution into "microevolution" (evolution of subspecies or geo- 

 graphic races) and "macroevolution" (evolution of species, genera, and 

 so on). He contended that, while "little" mutations can provide the raw 

 materials for the degree of diversity represented by subspecies, mutations 

 of an entirely different order of magnitude ("systemic mutations") must be 

 invoked to explain macroevolution (p. 503 ). The origin of large evolution- 

 ary changes receives further attention later (pp. 502-506). We may note 

 at this time, however, the difficulty of distinguishing clearly between 

 "little" and "systemic" mutations. Is a change in a single gene controlling 

 the rate of growth of a part of the body a "little" mutation or a "systemic" 

 mutation? Perhaps we may best describe it as a little mutation with a 

 big effect. It seems unlikely that a sharp line can be drawn between 



