ASPECTS OF KINETIC EVOLUTION 259 



grazing animals, especially when it is borne in mind that New 

 Zealand never contained such, excepting the various species of 

 Mo a." 1 



That the spines did not develop under conditions of moisture 

 and feeble light can scarcely be accepted, however, as proving 

 that they are a special contrivance for checking transpiration, 

 for many analogous adaptations do not fail to appear in advance 

 of the conditions which require them. Cacti, and other spiny 

 plants often make most of their growth in periods of humid 

 weather, but they do not on that account fail to put on spines. 



The possibility that the spines may be a useful form of tissue 

 for the plant when living in the normal desert habitat is not a 

 sufficient explanation of the failure to produce the spines under 

 conditions of humidity and deficient sunlight. The spines might 

 be an adaptive character and still appear under all conditions of 

 growth. They might represent an adjustment character or artism 

 and still be only reduced instead of being eliminated in the shade 

 form. That the spines disappear entirely indicates that another 

 factor may need to be recognized, that certain conditions are 

 necessary for their development, and that without these condi- 

 tions the plant is unable to make spines, just as the pepper-fed 

 canary birds may be thought of as no longer able to produce 

 yellow feathers. 



The interest of the Discaria experiment would have been 

 increased if it had included a test of the behavior of the plants 

 in shade conditions without excessive atmospheric moisture, to 

 determine whether deficiency of light might not of itself inhibit 

 the formation of the spines, simply by restricting the activity of 

 the cells. The formation of the spines is a specialization which 

 the seedling plants do not attain until they have grown to con- 

 siderable size, perhaps not until they have encountered condi- 

 tions of drought and exposure to strong sunlight. It is, there- 

 fore, not unreasonable to suppose that these conditions are a 

 necessity to enable the plant to produce the spines, and hence 

 that its failure to produce them represents not so much an accom- 

 modation as a lack of accommodation, that is, topism, instead 

 of artism. 



1 Cockayne, L., 1905. Significance of Spines in Discaria Toumatou Raoul 

 (Rhamnaceas), New Phjtologist, 4 : 79. 



