122 THE THEORY OF MUTATION 



(2) that such a form of variation is by no means so 

 uncommon a phenomenon as was formerly supposed ; 

 and (3) that variations of this kind may occur in 

 every description of organ and part in a number of 

 different plants and animals. The facts with which 

 the main bulk of the book is concerned have reference 

 to the animal kingdom. 



We shall find it profitable to consider the view r s 

 expressed in this book a little more closely, though it 

 would occupy too much space to give even a brief 

 summary of the facts upon which they are based, and 

 for which reference must be made to the original. 



In the first place Bateson calls attention to the 

 phenomenon of symmetry as being a characteristic 

 feature common to almost all organisms. This sym- 

 metry may manifest itself in a number of different 

 ways. In bilateral and radial symmetry the parts 

 symmetricaDy disposed are related to one another in 

 the same kind of way as are an object and its image 

 reflected in a plane mirror. Such symmetry, as, 

 indeed, every kind of symmetry, is usually associated 

 with a repetition of parts. In the present instances 

 the parts are repeated in pairs, as with the two eyes 

 in the human face ; or in a radial series, like the arms 

 of a star-fish, or the petals of a buttercup. To this 

 phenomenon of the repetition of parts, generally 

 occurring in such a way as to produce a symmetry or 

 pattern, the term merism is applied. 



Symmetry may affect the proportions and shape 

 of the body of an animal or plant as a whole, or, on 

 the other hand, separate parts or organs may show a 



