48 GERMINAL SELECTION. 



of the vital units by germinal selection. In a simila 

 manner are induced the most varied qualitativi 

 changes of the corresponding determinants and of th 

 characters conditioned thereby, just as changes in th 

 numerical proportions of atoms produce essentia 

 changes in the properties of a chemical molecule. 



In this way we acquire an approximate conceptioi 

 of the possible mechanical modus operandi of actua 

 events — ^namely, of the manner in which the usefu 

 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 abli 

 to understand how different parts of the body, abso 

 lutely undefined in extent, can appear as variationa 

 units and vary in the same or in different directions 

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

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

 case of the butterfly's wings it rests entirely witl 

 utility to decide the size and the shape of the spot 

 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 contrastei 

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

 trast will exist between the anterior and posterio 

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

 streaks will behave as variational units and form con 

 trasts with manifold kinds of spots or with the broade 

 intervals between them, with the result that the pictui 

 of a leaf or of another protected species is produced. 



I must refrain from entering into the details of sue 

 cases and shall illustrate my views regarding the coloi 

 transformations of butterflies' wings by the simplei 



