Species and Species Change 151 



to spring from a vertical surface. Later some such insects evolved 

 flight but only because they already had evolved locomotor mus- 

 cles near the anterior part of the body giving aerodynamic balance 

 and had strong legs giving propulsion at the take off. In the insects 

 the body balance and the few, strong legs were prerequisites to 

 the evolution of flight. 



A simple case can be postulated for the cats or felids involving 

 successive specialization of food. The early felids, as do many of 

 them today, probably fed on a variety of small foods such as in- 

 sects, small rodents, and rabbits (Hoffmeister and Mohr, 1957). 

 As the number of felid species increased and mixed in the same 

 habitat, food specialization undoubtedly occurred, the larger felids 

 eating the larger prey species, the smaller felids eating the smaller 

 prey. Subsequent to this food specialization, a selection for larger 

 size probably acted on the larger species because larger size would 

 enable the felid to catch full grown animals with their increased 

 food poundage where only juveniles had been accessible to the 

 felid before. Thus after the first food specializations, successive 

 sets of selection pressures moved the adaptive mode of some phy- 

 logenetic lines along a gradually ascending food gradient to points 

 far beyond the utilization bands of earlier stages of the lines. 



The examples of insect flight and felid food specialization involve 

 the evolution of prerequisite characters which have permitted phy- 

 logenetic lines to establish new adaptive relationships. Westoll 

 attributed the great burst of specialization of the dipnoian fishes 

 to the same processes. In many cases, as was indicated in Chapter 

 4, these changed phylogenetic lines become better and better 

 adapted to their new environment and finally become exti-emely 

 stable over long periods of time. 



REVERSIBILITY OF EVOLUTION 



In one sense, evolution is irreversible in that the changes in a phy- 

 logenetic line never change back to their original genetic state. 

 In an ecological sense, however, evolution is frequently reversed. 

 Both the dogs or canids and the cats or felids offer examples. The 

 canids became specialized for open country and running down 

 prey, but in the fox branch of the family at least one line, rep- 

 resented by the American gray fox, has reversed the group spe- 

 cialization and has become a forest animal with many cat-like habits 

 (Hoffmeister and Mohr, 1957). One line of the felids, represented 



