THE BUCKTHORN FAMILY 241 



wondered at, but the strangest part of the story yet remains 

 to be told. Dr. Cockayne has studied its development, and has 

 also shown by an experiment, which is probably destined to 

 become classic, that Discaria was originally a spineless leafy 

 plant adapted to a moist habitat.* The seedling plant is erect, 

 leafy, and bears no spines. After it attains an inch or two in 

 height, spines begin to develop in the leaf axils, and the 

 foliage becomes gradually sparse. If, however, the plant is 

 now put into a warm moist chamber, no more spines will be 

 developed, the leaves will be retained, and, — in a word, — the 

 plant returns to its seedling form. Nor is this a temporary 

 change, for this form will be retained as long as the plant 

 remains in an atmosphere saturated with moisture. And in 

 this complete suppression of the spines. Dr. Cockayne's 

 experiment is unique. Goebel, perhaps the greatest living 

 botanist, recently said : — " I do not think that up till now any 

 more has been proved, than that in moist air the formation of 

 prickles and thorns is retarded ; there is no proof that it can 

 be suppressed."! Dr. Cockayne seems clearly to have shewn 

 that complete suppression is possible. 



Such a remarkable experiment as this cannot fail to be 

 profound and far-reaching in its effect on biological ideas of 

 the species. As will be pointed out {v. Plagianthus hetiilinus, 

 p. 256), the seedling often passes through the ancestral forms 

 of the species in its development. We must, therefore, assume 

 that Discaria had originally small thin leaves, adapted to a 

 moist climate, but has changed its characters in response to 

 the stimulus of a drier atmosphere. Sixty years ago the 

 dogma of the fixity of species was one of the most sacred 

 beliefs of biologist and layman. Darwin showed us that 

 species are not fixed ; and now there are not wanting many 

 indications, which seem to prove, that in some cases, 

 individuals even, inay show a marvellous plasticity. 



*The New Phytologist Vol. IV. No. 4, On the Significance of the Spines in Discaria 

 tournatou. 



tPlant Organography Eng. Trans. Part I. pp. 263, 264. 



17 



