CHANGING ANIMALS 15 



If a species becomes extinct, the result is an environmental niche — a 

 possible place and means of livelihood — left vacant. There may be in 

 the vicinity, however, some other species possessed of structures which 

 preadapt it for life in the vacant "niche." For such a species the disap- 

 pearance of the former species would be the opening of a door to oppor- 

 tunity. Sometimes the environmental niche remains vacant for a long time 

 before a species appears that is adapted to occupy it. Thus Simpson 

 (1953) has pointed out that the niche left vacant by the extinction of 

 ichthyosaurs (reptiles, relatives of the dinosaurs, having highly fishlike 

 body form; Fig. 3.5, p. 30) was unoccupied until the advent of dolphins 

 and porpoises millions of years later. 



Accordingly we see that possession of bodily changes, the result of fortu- 

 nate mutations, may enable animals to meet changing environments in one 

 of two ways. ( 1 ) In some cases possessors of changes among members 

 of the species already present in the environment may be enabled to sur- 

 vive while their fellows cannot. (2) In other cases the species already 

 present in the environment may become extinct, but other species pos- 

 sessing structures which preadapt them to life in the niche left vacant 

 may be enabled to move in and occupy that niche. 



A specific example may help to make clear the application of the 

 general principles we have been discussing. We have already referred 

 to the fact that the crossopterygian fishes gave rise to the first land verte- 

 brates, the amphibians. This was one of the greatest changes to occur in 

 the evolution of vertebrates. More information concerning it will be found 

 in Chapter 8. This change occurred in the period of geologic time known 

 as the Devonian (p. 137). Prior to that time all vertebrates had been 

 water dwellers. Hence the dry-land environment was an unoccupied en- 

 vironmental niche, as far as vertebrates were concerned. As nearly as we 

 can picture it from our great distance in time, the course of events ran 

 somewhat as follows. 



Elevation of the land was reducing more and more the size of the lakes 

 and estuaries in which fresh-water fishes were living. During the dry sea- 

 sons some ponds probably dried up completely, while others were re- 

 duced to stagnant pools of foul water, overcrowded with fishes. Under 

 such conditions most fishes must have died, as they do when similar con- 

 ditions arise today. But among the fishes in those Devonian ponds were 

 some which were preadapted for invasion of the unoccupied environ- 

 mental niche just across the water line. These were the crossopterygian 

 fishes we have mentioned. Three of their most striking preadaptations 

 were: (1) the skeletal structure of the fins, providing raw material for 

 a limb that could support the body and accomplish locomotion when the 



