28 Arber — Atavism and Law Irreversibility. 



tinues automatically. Farmer's conception of the pro- 

 cess of atrophy is interesting* in connection with a 

 passage in "The Origin of Species" expressing the diffi- 

 culty of accounting for the complete loss of rudimentary 

 organs. "After an organ has ceased being used," Dar- 

 win wrote, "and has become in consequence much 

 reduced, how can it be still further reduced in size until 

 the merest vestige is left ; and how can it be finally quite 

 obliterated. It is scarcely possible that disuse can go on 

 producing any further effect after the organ has once 

 been rendered functionless. Some additional explana- 

 tion is here requisite which I cannot give." It seems 

 fairly obvious that any attempt to explain away this 

 difficulty on the Natural Selection hypothesis must be 

 artificial and inadequate, but Dollo 's principle, and espe- 

 cially that particular aspect of it at which Farmer has 

 independently arrived, at least brings the phenomenon 

 under a general law. 



It has recently been suggested to me by Dr. E. G. Salis- 

 bury that resupinated leaves, such as those of Allium 

 ursinum, Alstroemeria, etc., provide a clear instance of 

 the tendency on the part of the plant to continue along 

 any path on which it has once started. He points out 

 that we must suppose, on grounds of comparative 

 anatomy and morphology, that the Alstroemerias origi- 

 nally had normally orientated leaves. The first formed 

 leaves are held in the profile position, and it seems that 

 the plant, having begun to turn its leaves, has found it 

 easier to go on turning them than to turn them back, 

 despite the fact that the continuation of the original line 

 of evolution involves the differentiation of the morpho- 

 logically lower side as palisade, and the original palisade 

 as spongy tissue. I have been interested to find that 

 Salisbury's mode of visualising the process of leaf inver- 

 sion, as a commitment on the part of the plant to a course 

 from which there is no turning back, receives indepen- 

 dent confirmation in Dr. Lindman's 5 description of the 

 peculiar behaviour of a certain Bomarea in which the 

 leaves are invariably resupinate. He observed that the 

 mature shoots of this plant were often so placed that the 

 resupinate leaves would be 'upside down,' i. e., with their 

 morphologically upper side again actually uppermost. 

 But such leaves were invariably found to return to their 



5 Lindman, C. A. M., Zur Morphologie und Biologie einiger Blatter und 

 belaubter Sprosse, Bihang Svenska Vet. Akad. Handl., 25, Afd. Ill, No. 4, 

 63 pp., 20 text figs., 1899. 



