2122 



A TEXTBOOK OF THEORETICAL BOTANY t 



from the latter. The proposition is, of course, a mental concept, but it is 

 drawn from a considerable body of observed data and its truth is highly 

 probable. This is scientifically admissible but it is quite a different thing to 

 translate the proposition into terms of Sippen Phylogenie and deduce there- 

 from that A, which is sympetalous, has been derived from B, which is 

 apopetalous. On the contrary A may be actually less advanced than B 

 in respect of other equally important characters. This statement of the 

 position is perhaps unduly simplified, but it serve to illustrate the fallacy 

 inherent in phylogenetic classification, which seeks to reduce to linear order 

 something which is, in fact, a complex reticulum. The river of evolution 

 has not advanced with the uniform sweep of a tidal wave, but with multi- 

 tudinous cross-currents and eddies, which logic may analyse but can never 

 simplify without loss of truth. 



Classification presents itself differently to the palaeontologists. With 

 all its imperfections the geological record does show a real sequence of 

 organisms in time which is, at any rate in broad outline, an evolutionary 

 succession. It is here that Huxley's dictum that the problem of systematics 

 is that of detecting evolution at work is most clearly true, whatever 

 reservations we may put upon it in the field of living organisms. The 

 relationships which the succession reveals are more important to the 

 palaeontologist than the differences on which systematic distinctions rest. 

 He has to work with only a small part of the data which are available to 

 other biologists and his views on phylogeny are therefore bound to be 

 tentative and alterable. If the classification he uses depends upon his view 

 of the phylogeny of the organism concerned, then every change in ideas 

 about one produces consequent changes in the other, with bafiling results 

 in nomenclature. The lineages which are revealed by the succession of 

 fossils in time may be accepted, though never proved, as being genealogical, 

 that is truly phylogenetic. If a classification is to express this it must be 

 " vertical " in its groupings, but there is so much uncertainty in the 

 mterpretation of lineages that a " horizontal " classification of contemporary 

 forms is often the only workable method. In spite of the difference in 

 approach the same dilemma reveals itself, the incompatibility of a practically 

 workable classification and of an ideally phylogenetic classification, which 

 demands minute and prolonged study and often inaccessible data. 



Nowhere is the cleavage between the two aspects of taxonomy more 

 clearly revealed than in the vexed question of the nature of species. This 

 is not the place to discuss the origination of species, a complex problem 

 which we shall discuss in Volume HI, but we must consider here the ideas 

 of the species which underlie taxonomy. 



Not only is there no uniformity in the idea of the species, there is not 

 even consistency. Tate Regan defined a species as "a community or 

 number of related communities whose distinctive morphological characters 

 are, in the opinion of a competent systematist, sufficiently definite to entitle 

 it, or them, to a specific name". Darlington, on the other hand, has defined 

 species as " the minimum permanently isolated groups", adding the rider 



