OUR FUTURE COURSE 217 



a consideration of major steps in evolution, upon 

 finished characters, rather than upon the process 

 by which they are formed. In other words, we 

 judge the results of evolution as they stand before 

 us, and not the possible steps which preceded their 

 consummation. It is obvious that a hand may not 

 again become a fin, and that a flipper may not be- 

 come a foot, but quite true that an organism which 

 has legs may be the descendant of legless ancestors 

 and the progenitor of legless descendants. Each 

 structure is a positive phenomenon, the product 

 of a given heritage, and because of its positive 

 characteristics it is obviously impossible for it to 

 present the conditions of heritage necessary for the 

 production of a preexisting stage of its develop- 

 ment. As a result it cannot be the cause of its own 

 cause nor of something exactly like it, but within 

 its own latitude of development there is no funda- 

 mental obstacle to its fluctuation. 



As an example the probable history of the penta- 

 dactyl appendage is excellent. According to the 

 accepted view of the evolution of the terrestrial 

 vertebrates, the initial steps in the development 

 of these appendages occurred in the Crossopterygii 

 of the geological past. The skeletal elements were 

 present in these fishes, which apparently used their 

 pectoral fins as supporting structures when restmg 

 on the bottom. Here are environmental factors 

 common to terrestrial and aquatic habitats: the 



