154 MORPHOLOGICAL DEVELOPMENT. 



drawn to exemplify another truth, serves also to exemplify this 

 truth: the homologous parts a, b, c, d, while they are unlike 

 one another, are, in their main proportions, severally like 

 the parts with which they are paired. And here let us not 

 overlook a characteristic which is less conspicuous but not 

 less significant. Each of the lateral wings has winglets 

 that are larger on the one side than on the other; and in 

 each case the two sides are dissimilarly conditioned. Even 

 in the several components of each wing may be traced a like 

 divergence from symmetry, along with a like inequality in 

 the relations to the rest: the proximal half of each leaflet 

 is habitually larger than the distal half. In the leaves of 

 the Bramble, previously figured, kindred facts are presented. 

 How far such differences of development are due to the posi- 

 tions of the parts in the bud; how far the respective 

 spaces available for the parts when unfolded affect them; 

 and how far the parts are rendered unlike by unlikenesses 

 in their relations to light; it is difficult to say. Probably 

 these several factors operate in all varieties of proportion. 

 That the habitual shading of some parts by others largely 

 aids in causing these divergences from symmetry, is very 

 instructively shown by the compound leaves of the Cow- 

 parsnip. Fig. 211 represents one of these. While the leaf as a 



m 



whole is bilaterally symmetrical, each of the wings has an un- 

 symmetrical bilateralness : the side next the axis being larger 

 than the remoter side. How does this happen? Fig. 212, 

 which is a diagrammatic section down the mid-rib of the 

 leaf, showing its inclined attitude and the positions of the 



