502 TEXTBOOK OF BOTANY 



recognizing hereditary differences that have occurred and survived, yet 

 they cannot possibly be of any consequence in the survival of the species. 

 In view of the fact that innumerable hereditary characters that are of 

 no survival value to the plant have originated by exactly the same kind 

 of processes that initiate valuable characters, "survival of the fittest," if 

 it does occur, must be limited to very special cases. The teiTn "valuable" 

 in the above sentence refers to those characteristics that are essential to 

 existence, or to those that enable plants to grow in different kinds of 

 environments. 



Finally, a species, as generally conceived, is a mixed population of 

 very similar individuals, differing slightly from each other in gene com- 

 plement. If different individuals of a species become separated, or iso- 

 lated, in places where they no longer interbreed, they will in time be- 

 come sufRcientlv different to be regarded as different races or varieties. 

 Such race differences as have been intensively studied seem to bear no 

 relation whatever to survival, for they occurred in various local areas 

 having similar environments. Yet this differentiation of races is undoubt- 

 edly an initial step in the evolution of new species. The decisive fact is 

 that the indivduals became isolated from each other, and the different 

 kinds of mutations that occurred remained segregated in the local areas 

 because of the absence of cross-fertilization. 



Some of these racial differences do influence fundamental processes 

 such as photosynthesis and respiration; and if the environment changes, 

 some of the races may survive better than others, and some may be com- 

 pletely eliminated. One mav refer to the races that survive as examples 

 of the "survival of the fittest." Which of the races will be the fittest, how- 

 ever, depends upon the nature of the changes in the environment. The 

 races that are destro\'ed bv one kind of change in the environment misiht 

 be the fittest of the population if some other kind of change had oc- 

 curred. There seems to be no serious objection to the use of the expres- 

 sion "survival of the fittest" in this specific sense. One may rightly object, 

 however, to the idea that the heritable changes by which the different 

 races originated "occurred to meet the change in environment," or that 

 they were actually "selected by nature." To accept this last idea, one 

 would have to assume that the environment changed in order to favor 

 certain races and exterminate others. 



Another interesting point about the development of these isolated races 

 is the fact that some of the heritable changes that occur in them are 

 similar, or parallel, while others are di\ ergent. Since the individuals that 

 became isolated in different regional areas belonged to the same species. 



