ORGANIC EVOLUTION 



253 



Retrogression with change of function. — When a waning 

 organ loses its original function, it may be saved by being 



put to a new use. Thus certain 

 of the smallest innermost 

 branches of the our common 

 hawthorns have a brief exis- 

 tence as leafy branches and be- 

 come transformed into stout de- 

 |/*^-— ^iiw j| '-^g^,,^^M| fensive spines (fig. 156) which 

 r ■ W^^t^S^Lt^^^Kk then serve the tree by oppos- 

 ing the browsing of cattle. 

 The vanishing fifth stamen of 

 Chelonehas, in the allied genus 

 Pentstemon, ceased to be a poll- 

 en bearing organ, but has be- 

 come extraordinarily developed 

 to aid in pollen distribution (fig. 

 157). It is declined across the 

 base of the other stamens, 

 elongated and protruded in such 

 position that the entering bee 

 walks over its hairy tip, and in 

 so doing shakes the pollen from 

 the anthers of the other sta- 

 mens down upon its own back. 

 Specialization by reduction. 

 — It is important to note that 

 the dwindling and loss of parts 

 is generally a gain to their 

 possessors. Organs are of no 

 moment except as they serve 



Fig. 156. Hawthorn spines in the , . ,^, , 



making, a, compound spines, the organism. i he thrcc scrics 



formed from branches bearing r • - i • i 



well developed leaves; 6, simple OI mSCCtS WhOSC WmgS WChaVC 

 spines formed from short branch- , ^^ ^ ^^ i i •1-^ 



es with vestigial leaves. Studied all show decided 



