MEANS AND METHODS OF EVOLUTIONARY CHANGE 359 



As before, we may postulate that the first ancestral horses to venture 

 forth from the woods onto the plains were chased by predatory animals. 

 The action of the predators would constitute one of the checks upon the 

 too rapid increase in numbers of these ancestral horses. In the resulting 

 competition to survive and leave progeny which individuals would 

 succeed? If among the population of ancestral horses a mutation arose pro- 

 ducing longer legs (just the reverse of the Ancon ram mutation), posses- 

 sors of that mutation might be able to run faster than could their fellows. 

 If so, a disproportionately large number of horses not possessing the mu- 

 tation would become food for the predators before they had opportunity 

 to mate and reproduce. Thus the horses having the mutation for longer 

 legs would produce more than "their share" of offspring, with the result 

 that more of the next generation would inherit longer legs than possessed 

 them in the parental generation. If selection continued in the same way 

 for several generations the shorter-legged horses might disappear entirely, 

 leaving the field to the possessors of the longer legs. If, now, a second 

 mutation occurred, increasing the length of the legs still more, possessors 

 of that second mutation would be favored in the "strusgle for existence," 



OCT ' 



with the result that some generations later all horses would have the second 

 mutation, possessors of the first mutation having been eliminated. And so 

 step by step the progressive lengthening of leg observed in the evolution 

 of the horse might be explained through the operation of natural selection 

 on successive mutations. (For an alternative explanation for this example 

 see pp. 412-413.) 



In the preceding example we have kept the account as simple as possi- 

 ble in attempt to paint the broad outlines of the picture without including 

 confusing details. Actually the situation at any time would have been much 

 more complex, many factors in addition to length of leg entering into the 

 determination of which individuals should contribute most to subsequent 

 generations (see pp. 356-357). 



A Glimpse of Variables in the Process 



We have seen that mutations occur at definite, though usually low, rates. 

 Diff"erent mutations have different rates of occurrence (''mutation pres- 

 sures"). Thus the rate at which the raw materials of evolutionary change 

 are supplied varies for different mutations. 



The intensity of natural selection ("selection pressure") varies greatly 

 from time to time and from place to place. Under some conditions "the 

 living is easy"; under others survival and reproduction are extremely 

 difficult. 



