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JOURNAL OF THE ROYAL HORTICULTURAL SOCIETY. 



environments consist of an infinite variety of combinations of the relative 

 amounts of light, heat, and moisture of the air, as well as the composition 

 of the soil or water in which plants may live. Plants are also modified 

 so as to meet mechanical strains and stresses. In the Tropics one or more 

 of the^e agencies are permanently in excess ; hence a constantly high degree 

 of heat and moisture and a relative increase of nitrifying microbes are the 

 immediate causes of the dense tropical forests ; whereas great heat with 

 a deficiency of water results in the desert flora of a totally different 

 character. 



A plant has many functions to perform in order to live a healthy life, 

 and to set an abundance of seed. Its various organs, root, stem, leaves, 

 flowers, and fruits, require an optimum degree of heat, light, &c, re- 

 spectively, but plants rarely, if ever, secure a totality of such optima. 

 Thus, an ordinary flowering plant may thrive well enough in the shade, 

 but is incapable of flowering there ; conversely, on the sunny open downs 

 the plants may blossom freely enough, but remain dwarf in size. 



If we ask how a plant can become adapted to a changed environment, 

 there is as yet no answer ; for it is a property of life, and the properties 

 of life are inscrutable. The process, however, may be described as 

 follows : — As long as a species lives, generation after generation, in pre- 

 cisely the same average conditions, it has no inducement to change ; but 

 if the seeds of a plant be sown in a markedly different environment, then, 

 as they grow up, they may show a tendency to depart from the structure 

 and form of the parent plant, and to be more in harmony with the new 

 surroundings. Then, if their seedlings grow up under the same con- 

 ditions, a fresh increment is added to the alterations acquired. If this be 

 continued for some five or six years, experiments prove that a relatively 

 permanent form is reached in adequate equilibrium with the conditions 

 of life. The change of form., then, becomes relatively fixed and hereditary ; 

 and it may be reproduced subsequently even when the plant is raised in a 

 totally different environment. 



It must be borne in mind that more seeds are formed by a plant than 

 can possibly survive.* But many of the young plants die prematurely, 

 partly by being crowded out by those of better nourished seeds, partly by 

 being on a barren soil, while many are eaten off, &c. Such is the true 

 sphere of Natural Selection, but it has nothing to do with the origin or 

 survival of new varietal structures in the sense of Darwinism. These 

 always arise by response to what Darwin called the "definite action" of 

 the environment. Natural Selection represents the result of the struggle 

 for existence which goes on everywhere, but it is solely concerned with 

 the distribution of organisms. 



The principal argument for Adaptation is based on inductive evidence, 

 in that when we find a large number of plants, often having no affinities 

 whatever between them, to have assumed the same special adaptive 

 structures under the same or similar conditions of life so that they could 

 not have been derived by heredity one from another, it becomes a moral 

 conviction that each genus or species, as the case may be, has acquired 



* An accidental plant of the White Melilot came up in my garden. It bore nearly 

 16,000 seeds ; they would have required more than an acre if sown. A Foxglove 

 bore half a million of seeds. 



