42 STUDIES IN EVOLUTION 



VI. Prolonged development under conditions favorable for 

 multiplication. (Bi.) 



VII. By repetition. (B 2 .) 



VIII. Restraint of environment causing suppression of 

 structures. (Ci.) 



IX. Mechanical restraint. (C 2 .) 



X. Disuse. (C 3 , D 2 .) 



XL Intrinsic suppression of structures and functions. (Di.) 



To illustrate the various causes of spine growth, represent- 

 ative examples which are believed to conform to the require- 

 ments will be selected from various groups of organisms. 

 The number of spinose forms is so great that it will be impos- 

 sible to give more than the briefest citation of a few of the 

 leading types, especially those which have come under the 

 notice of the writer; on this account the number of examples 

 derived from the vegetable kingdom will be necessarily few. 



I. In response to stimuli from the environment acting on 

 most exposed parts. (A^) 



The action of external stimuli falling on the most exposed 

 parts of organisms is probably one of the most fundamental 

 and fertile causes of spine production, since the relation 

 between cause and effect is more direct and apparent here 

 than by other modes of origin. In a general way it com- 

 prehends all the remaining causes coming under the head 

 of external stimuli, but for present purposes it will be 

 restricted by the elimination of secondary conditions, such 

 as the indirect production of spines through differentiation 

 of previous structures, and the action of external forces of 

 selection. 



The ruling forces in plants being so largely vegetative, or 

 those of growth, and the cause of variation being principally 

 physico-chemical and not molar, most of the modifications 

 to produce spines will fall under other categories of origin 

 (B, D) than the one now under discussion. 



In the free swimming forms, however, as the desmids and 



