CLASSIFICATION OF PLANTS BY EVOLUTION. 



133 



present, their genealogy would prove to be as elaborate as the branching 

 and twigs of a large tree. Only, instead of reproducing the same form 

 over and over again, as of the flowers of a tree, all the buds of each year 

 in succession must be imagined to be different from those of the pre- 

 ceding year on the same branch. For variations of structure arise in 

 consequence of the seedlings or young animals varying as they grow to 

 maturity. This they do (if they vary at all) on a change of environment 

 or of "the conditions of life," as Darwin frequently expresses it. 



The process we now accept was described by Darwin as follows : 

 " If we ask ourselves why this or that character has been modified under 

 domestication, we are, in most cases, lost in utter darkness. Many 

 naturalists, especially of the French school, attribute every modification to 

 the monde ambiant, that is, to a changed climate, with all its diversity 

 of heat and cold, dampness and dryness, light and electricity, to the 

 nature of the soil, and to varied kinds and amount of food. By the 

 term 1 definite action ' I mean an action of such a nature that when 

 many individuals of the same variety are exposed during several genera- 

 tions to any change in their physical conditions of life, all, or nearly all, 

 the individuals are modified in the same manner. A new sub- variety 

 would then be produced without the aid of natural selection." * 



As soon as the offspring are dispersed — for migration, as a rule, is 

 essential — evolution begins. It is due to these two factors : (1) the direct 

 action of external forces of the environment, such as food, climate, &c, or 

 the monde ambiant, upon the organisms, and (2) a responsive power, which 

 the living protoplasm possesses, so as to be influenced by them. The 

 protoplasm and its nucleus then build up cells, the cells form tissues, and 

 the tissues organs in direct adaptation to the new conditions of life. 



Thus the plant shows differences in roots, stems, leaves, &c. from its 

 parent. 



In the next place, having acquired new structures, if the plant and 

 its descendants continue to live for several generations under the same 

 conditions and influences, the characters become "fixed," as horticul- 

 turists well know, and then 'they become hereditary, until, of course, 

 fresh changes induce new variations to arise ; but they will be, in part, 

 superadded on the modifications of the previous ones. 



* An. and PI. under Dom. vol. ii. p. 271. With reference to " Natural Selection " 

 it is true that Darwin thought that the action of the environment was generally 

 " indefinite " and not " definite," as described above ; by which he meant that all or 

 most of the offspring of a plant or animal would vary in all sorts of ways, or " indis- 

 criminately " as Romanes called it; so that one, two, or few individuals which were 

 perchance better adapted to the new conditions would survive, while all the rest 

 would perish. 



This was called the " struggle for existence, the fittest only surviving." Unfor- 

 tunately no evidence has ever been forthcoming in nature in support of this latter 

 view ; whereas the former, or the " definite action " of the environment, is universally 

 true. The real function of natural selection is the bringing about the Distribution of 

 Animals and Plants. Thus, if the seeds of, say, fifty different plants be sown 

 together, an intense struggle for existence takes place among the seedlings ; many of 

 the weaker ones die. In a few years not half that number of species will have 

 maintained their existence. 



In this struggle for life it is those with the strongest constitutions which are " best 

 fitted to survive," as they will have ousted the others That is all natural selection 

 can do ; but it takes no part in the origin of new variations of structure, upon which 

 alone varieties and species are based. 



