150 MORPHOLOGICAL DEVELOPMENT. 



are normally equal, or c and d, which form another pair noi 

 mally equal. 



Let us not omit to note, while we have this case before us, 

 the proof it affords that these differences of development are 

 in a considerable degree determined by the different con- 

 ditions of the parts after they have been unfolded. Though 

 those inequalities of dimensions whence the differentiations 

 of form result, may be in many cases largely due to the 

 inequalities in the circumstances of the parts while in the 

 bud (which are, however, representative of inequalities in 

 ancestral circumstances) ; yet these are clearly not the sole 

 causes of the unlikenesses which eventually arise. This bi- 

 lateralness resulting from the unequal sizes of the leaves, 

 must be considered as due to the differential actions that 

 come into play after the leaves have assumed their typical 

 structures. 



§ 225. How, in the arrangement of their twigs and leaves, 

 branches tend to lapse from forms that are approximately 

 symmetrical to forms that are quite asymmetrical, need not 

 be demonstrated: it is sufficiently conspicuous. But it may 

 be well to point out how the tendency to do this further 

 enforces our argument. The comparatively regular budding- 

 out of secondary axes and tertiary axes, does not usually 

 produce an aggregate which maintains its regularity, for 

 the simple reason that many of the axes abort. Terminal 

 buds are some of them destroyed by birds; others are bur- 

 rowed into by insects; others are nipped by frost; others 

 are broken off or injured during gales of wind. The envi- 

 ronment of each branch and its branchlets is thus ever 

 being varied on all sides: here, space being left vacant by 

 the death of some shoot that would ordinarily have occupied 

 it; and there, space being trenched on by the lateral growth 

 of some adjacent branch that has had its main axis broken. 

 Hence the asymmetry, or heterogeneity of form, assumed 

 by the branch, is caused by the asymmetrical distribution 



