CONSTANCE: SYSTEMATICS OF THE ANGIOSPERMS 409 



nized by followers of the natural systems. It was now clear that "affinity" could 

 be explained only in terms of actual genetic relationship, and that any natural 

 classification must be based upon lines of evolutionary development. As a re- 

 sult of this stimulus there began to emerge the ostensibly phylogcnetic systems, 

 which hold the field at the present time. Although Hooker was convinced of the 

 truth of evolution by modification, Bentham was not fully converted when work 

 on the Genera Plantarum got under way, so no mention was made of the theory 

 in that classic. 



Phylogenetic Systems 



The change from natural to phylogenetic systems of classification was not a 

 sharp one, but a scarcely perceptible transition in many cases. It is difficult to 

 determine whether certain classifications were intended by their authors to be 

 evolutionary or not, and there are differences of opinion in this respect with 

 regard to the underlying philosophy of even the Engierian "Principles" (Blake, 

 1935; Lawrence, 1951). This system, like that of Warming's, was based on the 

 arrangement of Eichler (1875); Eichler's, in turn was based on that of Braun, 

 who was opposed to the theory of descent (Baron, 1931). Thus, Turrill (1942, 

 p. 671) says, "Engler's system does not claim to be phylogenetic in the complete 

 sense but to show in its sequence of groups progressive complexity of structure, 

 apart from accepted subordinate reductions." On the contrary, I am inclined 

 to believe (on the basis of my own rough translation of the PrincipJes of Syste- 

 matic Arrangement, Engler, 1936) that Engler did intend his system to be phy- 

 logenetic, if due allowance is made for his concept of an essentially autonomous 

 origin and evolution for almost every plant family. These begin with the state- 

 ment {ibid., p. ix) : 



The endeavor of the scientific classification of plants, or systematic botany, is directed 

 chiefly toward grouping plant forms according to their natural relationship into assem- 

 blages of lower and higher grade. . . . When natural relationship is spoken of here, this 

 is an undoubted redundancy, for relationship in the true sense of the word is always a 

 natural one. 



Immediately thereafter follows a criticism of the older systematists for misus- 

 ing the term "relationship" to cover instances of mere similarity in a given fea- 

 ture, as opposed to "actual genetic relationship" expressed by agreement in 

 ontogeny and anatomy of organs, chemical characteristics, and the possibility 

 of a common origin in the same part of the earth. Engler emphasized the great 

 diversity of unicellular organisms and their manifold developmental tendencies, 

 and suggested that the various stocks of living species were early differentiated 

 from each other genetically and separated from each other geographically. He 

 stressed parallel courses of evolution, and the danger of mistaking analogies, 

 i.e., the attainment of comparable evolutionary stages in different structures, 

 for evidence of genetic affinity. Species were stated to be capable of giving rise 

 to divergent descendants or of mutating, presumably in parallel directions un- 

 der similar conditions at different places. These ideas are extended to suggest 

 the possible origin and development of completely distinct stocks — the concept 

 of polyphyletic origin of angiosperms. 



The problem of scientific systematics is, however, not merely to unite the forms dis- 

 tinguished by common traits with groups of lower or higher rank, but it must strive 



