244 



ESSENTIALS OF BIOLOGY 



Greenland cod is 53-18 and it will be seen that there are 119 

 variants in the class 53, that is the class nearest to this mean value. 

 Now when we merely observe, but do not make experiments 

 upon naturally occurring populations, or races, of organisms we 

 see such crude variability, in respect of all structural characters 

 that can be measured. We can, in nearly all cases, represent 

 this observed variability by such frequency distributions and 

 graphs as have been instanced above. 



^% 80- 



<^JL 40- 

 ^ 20- 



50 61 52 53 54 

 Nos. of uertebrou& 



MeaiL value 

 of the chjaroLcterlstijc 



Fig. 34. 



I, A histogram ; 2, a frequency curve. 



846. The Analysis of Crude, Organic Variability. How- 

 ever, it is possible to make analyses of the variability by subjecting 

 organisms to domestication, by selecting them, cross-breeding and 

 inbreeding them, etc. — in short, by exercising control over their 

 habits of reproduction and life in general. It is only practicable 

 to do this in the cases of organisms that may reproduce freely 

 in the artificial conditions of the garden, farm or laboratory, and 

 not many cases of crude organic variability have been examined 

 in such ways. But it is permissible to extend (with caution) the 

 relatively few results that have been obtained to variability in the 

 cases of populations living in the wild state. 



We distinguish, in the crude variability discovered by inspection, 

 the following kinds of variants : 



(i) Mendelian variants ; 



(2) Mutants ; 



(3) Random fluctuants and 



(4) Fluctuants by acquirement. 



