134 THE THEORY OF MUTATION 



many cases for instance, in the case of the rose arise 

 by the development of petal-like organs in the position 

 v.hich would properly be occupied by stamens. A 

 parallel process is to be seen in the heads of com- 

 posite flowers, such as the chrysanthemum. In a 

 double chrysanthemum the florets of the disc develop 

 in the likeness of ray florets. Both these cases would 

 be classed as examples of outward homceosis, because 

 the parts concerned resemble organs normally de- 

 veloped in a whorl exterior to themselves. A case of 

 inward honuzosis, on the other hand, is afforded by 

 the appearance of a petaloid calyx for example, in 

 a tobacco-plant the outermost whorl of the flower 

 taking on the appearance of a whorl internal to itself. 



In cases such as these we observe once more the 

 occurrence of a marked and definite change, which, 

 though at first sight quite distinct from the method of 

 similar and simultaneous variation, yet bears a certain 

 resemblance to that process in the fact that the direc- 

 tion in which a particular part varies is not wholly 

 unrelated to the behaviour of other parts of the same 

 organism. The process thus briefly described seems 

 likely to have had considerable importance in evolu- 

 tion, notably in the origin of differences in the numerical 

 relations of the bones in various parts of the spinal 

 column in different vertebrate animals. 



The preceding account of the conclusions drawn 

 from Bateson's laborious study of variation has in- 

 volved a good deal of technicality, but this is, un- 

 fortunately, unavoidable. The point chiefly to be 

 emphasized is the frequent occurrence in Nature of 



