$0 JOURNAL OF THE ROYAL HORTICULTURAL SOCIETY. 



Darwin wrote as follows to Prof. Moritz Wagner, of Munich : — " The 

 greatest mistake I made was, I now think, that I did not attach sufficient 

 -weight to the direct influence of food, climate, &c. quite independently of 

 natural selection. When I wrote my book [1859] — and for some years 

 later — I could not find a good proof of the direct action of the environ- 

 ment on the species. Such proofs are now plentiful." * Of course they 

 were as plentiful in 1859 as in 1876. In fact they are universal. I cannot, 

 therefore, but think that had Darwin lived till to-day he would have 

 learnt that definite variations form the universal law in nature, and that 

 natural selection is only concerned with the distribution of organisms 

 and not with their origin. As I have published two books t on this 

 subject I need only refer the reader to them for a full account of the 

 argument based on both inductive and experimental proofs. 



I would, however, like to add here a special case, as some experiments 

 have completely corroborated the inductive evidence recorded in the 

 second volume mentioned (p. 162). 



It is a general fact that submerged leaves of dicotyledons are finely 

 dissected ; in a few cases they are ribbon-like, but it is exceptional. The 

 former type is found in a great number of plants of quite distinct families ; 

 hence there is no affinity between them to account for it, and we are 

 compelled to see some connection, or " cause and effect," between sub- 

 mergence and the dissected foliage. 



A plant named Proserpinaca palustris of the United States belongs 

 to the order Haloragece, to which the Water Milfoil (Myriophyllum) and 

 the Mare's- tail belong, the former of which has dissected leaves and 

 lives submerged. Now, the Proserpinaca is amphibious, so that when the 

 leaves grow in air they are completely formed, being of a narrow, 

 lanceolate shape with a serrated margin ; but when it is submerged, the 

 plant has leaves consisting of the midrib and lateral veins only, so that 

 it is of a pectinate form. 



In order to ascertain the actual cause of the change, Mr. W. Burnet 

 McCallum experimented with light, nutrition, temperature, salts, relation 

 to C0 2 and 0 2 , moist air, and a constant stimulus. 



The conclusion was that he excluded everything but water as a cause, 

 as was previously inferred by inductive evidence ; but he went further by 

 adding substances to the water which withdrew the excess of water from 

 the protoplasm in the cells ; the Proserpinaca then at once developed the 

 air-form type of leaf under water. The following is Mr. McCallum's 

 description of the process : 



" The essential feature common to the water and moist air is the 

 inhibition of transpiration and the consequent choking of the cells and 

 the diluting of the protoplasm with water. This can be tested by 

 growing the plants entirely under water and at the same time drawing the 

 water out from the protoplasm, or in reality causing evaporation or transpira- 

 tion by means of high osmotic pressure. Plants producing water leaves were 

 placed in nutrient solutions of a strength not quite sufficient to plasmolyse 



* Quoted by Biichner in Last Words on Materialism, p. 194. It is also included 

 in his " Life " Ac. 



f The Origin of Floral Structures and Tlie Origin of Plant Structures (Inter- 

 'i.itionul Scientific Scrips, K. Paul, Trench & Co.). The first is abridged in Tlie 

 Making of Flowers (S.l'.C.K.). 



