Species and Species Change 139 



processes of genetic mutation and natural selection, one might sup- 

 pose that the variation in a species would be infinite, but this is 

 not the observed state of affairs. Simpson (1944) calculated the 

 coefficient of variation for several characters in diverse groups of 

 mammals and other animals and for 70 characters in man and 

 found that almost every case fell within a narrow range of varia- 

 tion. Exceptions to this low range of variation are well known, 

 especially in polytypic butterflies (Downey and Lange, 1956) and 

 in plants (Clausen, 1951; Stebbins, 1950). In the mammals Simpson 

 ( 1944 ) noted two examples of high variability, one the tail of the 

 opossum, another the length of a multituberculate tooth of a mam- 

 mal. Simpson believed this latter case to represent a degenerating 

 structure and hence one which might be expected to be unstable 

 genetically. In these examples of unusually great variation, it is 

 almost invariably only one structure which is involved— the wing 

 pattern of a butterfly, the leaf of a plant, the degenerating tooth 

 of a mammal, the tail of a marsupial. In the same species, literally 

 thousands of other characters will have a low value of variability. 

 Simpson's data ( 1944 ) bring out another important fact. Al- 

 though a given structure may evolve considerably within a phy- 

 logenetic line through a portion of geologic time, at any one time 

 during this evolution, the structure has the same small coefficient 

 of variation. 



THE FUNCTIONING PHYLOGENETIC LINE 



From these various considerations the species is seen to represent 

 a phylogenetic succession of parent-off^spring generations progress- 

 ing through time. At any point in time this phylogenetic line has a 

 dynamic range, a dynamic population density, and a dynamic ge- 

 netic constitution, all directly under the influence of a dynamic 

 environment. The compounding of these elements endows the phy- 

 logenetic line with tremendous possibilities for change. However, 

 the phylogenetic line may have a finite limit to its total amount 

 of genetic variability at any one time and concomitantly a finite 

 limit to its total phenotypic range of ecological tolerances and 

 adaptive adjustments. 



Adaptive Change 



Adaptation is expressed in some phylogenetic hues by great sta- 

 bility, in others by great change. On the basis of fossil data, Simp- 

 son (1944, 1953) demonstrated that evolutionary change is multi- 

 paced. There is every reason to believe that the same differences 



