TRANSFORMISM 243 



may be a matter for investigation in each case. The individuals 

 of the local races (in the well-known examples) can always be 

 recognized as belonging to the same species. Thus the North 

 Atlantic cod {Gadiis callarias) can always be immediately recog- 

 nized as cod, but there are the races, Greenla?idiais, Hibeniicus, 

 etc., and these are seen, upon inspection, to be cod that differ 

 from each other in some details of morphology. Further we 

 know that there is local segregation : that is, Greenland cod, for 

 instance, do not migrate into the Irish Sea and vice versa. 



In what follows we consider the local races, presuming that a 

 species has usually these sub-categories. 



84^. Organic Variability in Gener.\l. What we actually 

 find from mere inspection is that the individuals of a local race 

 are not similar to each other in all the details of their structure. 

 Thus Greenland cod have 51 to 55 vertebrae in their backbones 

 w^hereas Irish Sea cod have 50 to 54. If we examine a large 

 number of fish we find that the variability in respect of the struc- 

 tural character, number of vertebrce, has a certain form in the case 

 of each local race. Thus samples of Greenland and Irish Sea cod 

 were obtained and the figures in the tables (frequencies) show how 

 many fishes in a sample had 50 vertebrae, 51 vertebrae, and so on. 



Greenland cod. 



Frequency of occurrence of N. , .1 



A^ = number of vertebra . . • 51 



Irish Sea cod. 



Frequency of occurrence oi N. . . 5 



N = number of vertebrae . . . 50 51 5^ 53 54 



These series of figures are " frequency distributions." In 

 each of them we see that the character, " number of vertebras," is 

 variable. It may be from 51 to 55 in the first case and from 50 

 to 54 in the second one. These values express the " range of 

 variability," but it will be seen that the range is not quite the same 

 in the two examples. It is customary to graph such frequency 

 distributions as shown in Fig. 34 on page 244. 



The individual organisms that vary in respect of some structural 

 character are called varia?its and we see that 5 variants, all exhibit- 

 ing the same number of vertebras, are placed in Class 50, 62 in 

 Class 51, 114 in Class 52 and so on. The class that contains 

 the greatest number of variants is usually near the mean value 

 of variable character. Thus the mean number of vertebrae in 





