^ug. 3, 1876] 



NATURE 



29:^ 



ON THE CLASSIFICATION OF THE VEGE- 

 TABLE KINGDOM"- 



I. 



CLASSIFICATION is a natural propensity of the 

 human mind. If our attention finds itself directed 

 to a large number of objects, about which we desire to 

 inform ourselves, a desire to economise our labour, or 

 even render it possible, at once leads us to endeavour to 

 throw the assemblage into subordinate groups. The 

 result, and indeed end, of this process is to enable 

 us to frame general statements about these groups 

 which cover all the things comprised within them. In 

 the case of a naturalist it is desirable that the groups 

 should be so constituted as to admit of as many general 

 statements as possible being made with regard to them ; 

 and in proportion as our classification allows us to do this 

 successfully, we say it is a natural one — one conformable 

 to the order of nature — and such as nature herself would 

 indicate if the task were assigned to her rather than 

 undertaken by us. 



The question, however, immediately arises, What is 

 the cause which brings about this possession of common 



Figs. 1-5. — Development of colonies of Bac/erzumru6etcfHs alter hankesltr 

 (•' Quart. Journ. Micr. See," 1876, Plate III ). 



characters by each member of a group of organisms, and 

 renders their natural classification possible .-' We are 

 now able to answer with a very high degree of probability 

 of the explanation being the true one, " that propinquity 

 of descent — the only known cause of the similarity of 

 organic beings— is the bond, hidden as it is by various 

 degrees of modification, which is partially revealed to us 

 by our classifications."* 



The earliest attempts at classification seized upon the 

 most striking superficial distinctions. When Solomon 

 " spake of trees from the cedar tree that is in Lebanon, 

 even unto the hyssop that springeth out of the wall," it is 

 quite evident that mere size was the point of comparison 

 which aided the process of passing them under review. 

 And till the time of Ray and the beginning of the 

 eighteenth century the classification of plants into trees, 

 shrubs, under-shrubs, and herbs held its ground, though 

 nothing is now better understood than that size, which 

 is a mere matter of habit and mode of growth, is 



' Notes of four lectures delivered at the Royal Institution during February 

 and March. 



' Darwin, " Origin of Species," .jth Ed. p. 489. 



no clue at all to the real affinities of plants. It is easy 

 to see in point of fact when we have once grasped the 

 principle of descent as the cause of resemblances, that 

 those characters which are most valuable for classificatory 

 purposes, are generally those which are least prominent. 

 From age to age organisms may vary in response to the 

 changes of the external conditions to which they are ex- 

 posed. Nevertheless, underlying the most manifold modi- 

 fications, some apparently insignificant detail of structurtt 

 or development will be handed on unchanged, because it 

 has never happened to conflict with the stress of existence, 

 and such a detail will reveal the story of relationship 

 which the comparison of more striking, but really le.'^s 

 essential (because adaptive) external modifications would 

 perhaps completely obscure. 



Thus, comparing the two great departments of activity, 

 into which the life of plants is divisible — nutrition, i.e., all 

 that concerns the growth or multiplicatioa of the same 

 individual, and reproduction, i.e., all that concerns the 

 production of a new individual, while characters drawn 

 from nutritive structures (such as branching and tex- 

 ture of stems, form of leaves, &c.), have proved of little 

 value, those taken from reproductive structures have 

 proved of the highest importance for purposes of classi- 

 fication. And the reason is that a plant must live before 

 it can reproduce. The stress of competition is harder 

 on the nutritive side of its life than on the reproduc- 

 tive. Habit of growth, which is the expression of the 

 plant's attempt to adapt itself to the conditions of exist- 

 ence prescribed to it, must vary as the conditions vary ; 



Fig. 6. — Zooglcea. stage of BacieriutH ruifscens »ftcr Lankester ("Quart. 

 Journ. Micr. Soc," 1873, Plate XXIII.). 



but the development of ovules and homologous organs 

 comes when the battle of life, so to speak, is won. Their 

 details of structure, and the development of the embryo- 

 plants which proceed from them are, at any rate, in a 

 great measure relieved from the necessity of undergoing 

 adaptive changes. They undergo, no doubt, progressive 

 modifications, but these are comparatively slow and are 

 perhaps brought about in part by the correlation of 

 growth, which causes a changing part of an organism to 

 effect alterations in other parts which are not at first 

 implicated in or directly benefited by the original modi- 

 fication, and yet cannot help participating in it because 

 the organism must alter more or less as a whole. 



Thus, then, as amongst human beings, whether we 

 consider the family or the race, similitude or family like- 

 ness impHes blood-relationship or community of descent ; 

 in all organisms resemblances in structure which are 

 constant in large groups or vary very slowly, imply origin 

 from a common ancestor. The real problem of classifica- 

 tion is nothing less than to group organisms as we 

 should see them grouped if we could inspect the mighty 

 family trees of the plant or animal worlds. This mode of 

 regarding the facts of natural history is termed phylogeny. 



In undertaking the actual task of classification, we pro- 

 ceed on the assumption that as in a tree the twigs which 

 form the growth of any one year belong to branches of 

 all ages— from the very earliest to the very youngest — the 



