HOOKER LEOTURE, 1917. 111 
detailed Systematic Works the sole endeavour must be to arrange the material 
so as to indicate phylesis. 
It seems easy at the present day to grant this in theory, but it is difficult 
indeed to carry it out consistently in practice. For it involves the whole 
problem of Natural Relationships, which should be based upon the sum 
of all knowledge relating to the organisms classified. It is then little 
matter for surprise that the change of outlook, necessarily following on 
the acceptance of Evolutionary Theory, reacted very slowly upon the 
Classifications current during the second half of the 19th Century. We 
may take as an example the Magnum Opus of Bentham and Hooker, the 
‘Genera Plantarum,’ undoubtedly the greatest systematic work of the 
period. The object of the book was to formulate, on the basis of personal 
observation, the generic diagnoses of Flowering Plants. The grouping 
of those Genera into Orders, and the arrangement of the Orders among 
themselves were left substantially as in the systematic works current at 
the time: for instance, in the * Prodromus of De Candolle. The old Sub- 
Classes were retained. The Monochlamydez were kept apart as a separate 
Sub-Class, and the Gymnosperms were spliced in between the Dicotyledons 
and Monocotyledons. It does not follow, however, that this would be 
the considered opinion of the author of the Monograph on Welwitschia. 
It seems obvious that in a work, the value of which consisted in the 
definition of genera, the further problem of the grouping of those genera 
was not undertaken. The current groupings were retained so as not to 
raise questions apart from the immediate task. However deeply Sir Joseph 
Hooker’s mind was imbued with evolutionary belief, such views were not 
obtruded in the text of the joint authors. Nevertheless, those who study 
the last sections of the Ordinal Diagnoses of the ‘Genera Plantarum’ will 
find indications of affinity far in advance of the Classification which the 
authors had retained. But since the general scheme of arrangement 
followed that in vogue in Pre-Evolutionary days, the ‘Genera Plantarum’ 
may be classed as technically belonging to that period. We thus see 
that the effect of the Dogma of Constancy of Species, as reflected in 
Systematic books, lasted long after it had ceased to hold the field. The 
reason for this is to be found in the impossibility of remodelling the 
broader lines of classification until time had familiarized the new aspect 
of old facts ; and it was necessary to wait till the multitude of new facts 
essential for full argument should have been acquired. 
It might thus seem to the superficial observer as though Natural Classi- 
fication had benefited little by the advent of Evolution. The charge 
of ultra-conservatism, or even of inconsistency, might ignorantly be laid 
against great men of science, such as Bentham and Hooker; though the 
only possible ground for such a charge might actually be their unwillingness 
to force a change prematurely. This is probably the chief reason why, 
