48 GERMINAL SELECTION. 



of the vital units by germinal selection. In a similar 

 manner are induced the most varied qualitative 

 changes of the corresponding determinants and of the 

 characters conditioned thereby, just as changes in the 

 numerical proportions of atoms produce essential 

 changes in the properties of a chemical molecule. 



In this way we acquire an approximate conception 

 of the possible mechanical modus operandi of actual 

 events namely, of the manner in which the useful 

 variations required by the conditions of life can al- 

 ways, that is, very frequently, make their appearance. 

 This possibility is the sole condition of our being able 

 to understand how different parts of the body, abso- 

 lutely undefined in extent, can appear as variational 

 units and vary in the same or in different directions, 

 according to the special needs of the case, or as the 

 conditions of life prescribe. Thus, for example, in the 

 case of the butterfly's wings it rests entirely with 

 utility to decide the size and the shape of the spots 

 that shall vary simultaneously in the same direction. 

 At one time the whole under surface of the wing ap- 

 pears as the variational unit and has the same color; 

 at another the inside half, which is dark, is contrasted 

 with the outside half which is bright ; or the same con- 

 trast will exist between the anterior and posterior 

 halves; or, finally, narrow stripes or line-shaped 

 streaks will behave as variational units and form con- 

 trasts with manifold kinds of spots or with the broader 

 intervals between them, with the result that the picture 

 of a leaf or of another protected species is produced. 



I must refrain from entering into the details of such 

 cases and shall illustrate my views regarding the color- 

 transformations of butterflies' wings by the simplest 



