THE FOUNDATIONS OF GENETICS 265 



become known as time advances and genetics in the meanwhile 

 has given us a convincing account of the undc^rlying mechanism 

 of the entire process. 



Variation, one of the fundamental factors in adaptation, is 

 no less important in the science of genetics. Were it not for 

 variation we could have no knowk^dge of the methods of inherit- 

 ance. The fact of heredity would be no less evident; indeed, 

 absolute likeness of all individuals of a species through successive 

 generations would be even more definite evidence of its occurrence 

 than the partial resemblance which is known to occur. It is in 

 the variability of individuals, however, that the behaviour of 

 individual characters can be traced. When differences are mixed 

 in one generation and reassorted in the next they furnish a con- 

 trast in which the course of any one character can be seen and 

 traced. 



Kinds of Variations: Nature. In variation lies the range of 

 possibility of change within the organism. Extensive studies have 

 shown how universal variation is, and have given rise to a classi- 

 fication based on the nature, degree, heritability and evolutionary 

 tendency of variations. 



According to their nature variations are of three kinds: 



Morphological. Variations in structure may involve differences 

 in either the form or the size of parts, or in the case of duplicated 

 organs they may involve differences in the extent of duplication. 

 Differences in number are very common in plants. The petals 

 of flowers, lobes of leaves, leaflets, and other structures which 

 are usually duplicated may vary widely in number. In animals 

 such variations are less commonly available, but the radially 

 s.ymmetrical forms often have more or less than the normal 

 number of parts. Hydra with six or even seven tentacles in place 

 of the usual five and Asterias forbesii with four or six rays instead 

 of five are frequently encountered in the laboratory. In man 

 extra fingers or toes sometimes occur, a condition known as 

 Polydactyly (Fig. 154). Differences in form and size of parts 

 are evident to all of us in everyday contacts. It is more difficult 

 to find human beings with approximately the same appearance 

 than with very different appearances. 



Physiological. Variations in function are a necessary corollary 

 of morphological variations, since functions are merely the activi- 

 ties of structures. In the varied capacity of human beings to 



