Life is Flux 191 



facts in harmony with the assumption that individual vari- 

 ants capable of transmitting their distinctive characters had 

 become isolated in these various valleys and eventually sup- 

 plied the snail populations for them. There is not the kind 

 of difference between one valley and the next that would 

 call for specialized adaptation. The living conditions and 

 the food substances are substantially the same. The effect 

 here is as if a mutation had been segregated to prevent 

 swamping by the parental type. In the practical isolation 

 provided by the land formation hundreds of varieties have 

 been able to establish distinct populations. In a similar way, 

 each of the islands of the group contains species not found 

 in the others, and yet all are obviously derived from a 

 common source. 



Professor David Starr Jordan formulated some twenty- 

 five years ago, as a result of extensive regional studies of fish, 

 the following general principle: " Given any species in any re- 

 gion, the nearest related species is not likely to be found in 

 the same region, nor in a remote region, but in a neighboring 

 district, separated from the first by a barrier of some sort, 

 or at least by a belt of country the breadth of which gives 

 the effect of a barrier." Dr. Jordan's generalization was 

 based merely on facts observed in the field and is abundantly 

 supported by observations among many different kinds of 

 animals. At the time these observations were formulated 

 the distinction had not been clearly made between 

 genotypes and phenotypes, or between mutations and fluc- 

 tuating variations. Darwin had recognized that any new 

 character, however it might arise, would be in danger of 

 rapid elimination through swamping by the overwhelming 

 numbers of the conventional forms unless it were in some 

 way segregated. 



Life is Flux 



As we look upon the world of living things, we see it in 

 constant flux. Every individual changes as it lives. Growth 

 and maturity mean a developmental change. From moment 



