mechanism, respectively. Different criteria may have been taken by different writers, 

 but it is sufficiently clear that all three standpoints require to be considered together, 

 and the subject rapidly grows in complexity. 



One should always remember that in nature there are no genera or species, only 

 individuals ; classification represents the abstract ideas of human intelligence. Plants 

 which recognizably solve all the problems of somatic and reproductive organization 

 in the same way, and thus appear very much the same sort of thing, are included in 

 the same ' genus ', and the genus becomes the historical unit of classification. Minor 

 variants on the same methods, noticed in more detailed observation, delimit ' species ', 

 in so far as they may be constant, and such factors breeding ' true to seed '. Minor 

 variants, or mixtures which do not breed true, give ' varieties ', in different categories, 

 which may be increased by vegetative propagation; and any apparently constant 

 variation in a seedling (as a ' mutant ') may constitute what is to all intents a new 

 species, so far as constancy may be established. Experimental evidence is required 

 in any given case, and this is usually wanting or imperfect in the case of tree-types ; 

 e. g. the recognition of ' hybrids ' which have not been experimentally produced rests 

 on purely hypothetical considerations. We are still doubtful as to the time required 

 to change what is recognized at the present time as a good species into something 

 sufficiently different to be accepted as an equally distinct form ; and the time required 

 for the isolation of a new ' generic ' type is beyond human experience. 



Genera which agree in wider range of main principles are grouped vc\ families. 

 Families, in which common factors of still more fundamental significance can be 

 traced, are conceived as representing phyla of wider relationship. To do this it is 

 necessary to determine the essential value of the ' factors ' utilized for classification, 

 and this varies with the knowledge and mental outlook of successive generations of 

 ' systematists '. A broader outlook is always to be obtained by further investigation 

 of all plant-life, and not of one group alone. Hence systematists of a single group 

 (Monographers) are not expected to have a very wide or correct appreciation of the 

 factors with which they deal, and there is always room for improvement in any 

 provisional ' system '. 



Such generalizations express the manner in which systematy has been built up 

 along academic lines, as a matter of general history of the science, but other outlooks 

 are possible. Many botanists still fail to appreciate what is meant by the individual 

 life with which one deals in practical investigation. The individual is no longer to 

 be regarded as an isolated unit, or a casual creation, but is the present representative 

 of a 'race'. That is to say, the individual is not, as short-sighted chemical physi- 

 ologists tend to believe, a mere physical mechanism, the creature of the external 

 environment to which it passively responds ; but it is the living presentation of a 

 continuous line of organism, successful since living, or a ' race ' leading back as the 

 expression of continued response to very similar, but not necessarily identical, 

 environment, in unbroken plasmatic continuity, over a period of time which, in terms 

 of ultimate cytological history, may represent a continuous reaction and record for 

 anything up to such an inconceivable period as two thousand million years. During 

 this period the more fundamental reactions, as expressed in morphological units of 

 construction, have been established as constants beyond any hope of change. No one, 

 for example, now expects to find higher autotrophic plants expressed in any other way 

 than in terms of nucleated plasma with chloroplasts and wall-membranes of cellulose. 

 Even coenocytic organization is recognized as anomalous, and the rare expression of 

 somatic deterioration, usually heterotrophic. Similarly, at a later date, the general 

 features of stem (axis), leaf-appendages, phyllotaxis mechanism and ramification, as 

 well as details of vascular anatomy, have been elaborated and established by such 

 rigid natural selection that they express almost equally constant morphological factors, 

 to which any accidental exception appears as a ' monstrosity ' (cf. fasciation-effects, 

 phylloclades of Phyllocladus}. Even such primary factors of a land-plant may have 

 an age of over one thousand million years behind them ; and they are hence not 

 lightly changed. It is accepted, for example, that the construction of a forest timber- 

 tree, with minute anatomy and the attainment of the seed-habit at a horizon practically 

 equivalent with that of Ginkgo, dates to the Devonian at least, or a period of probably 

 over three hundred million years ; and these factors, again, are not lightly changed. 

 The loss of the seed-habit is practically inconceivable for higher plants (however 



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