PRINCIPLES OF MORPHOGENESIS 



15 



elusions of the experimental ecologists H. M. Hall and F. E. Clements, as related in their monu- 

 mental monograph of 1923.'" 



The basis of this monograph lies in "experimental and quantitative methods," and the experi- 

 ments to be conclusive "must extend over a long period of time." The authors have taken many 

 species of sagebrush and transplanted them from one environment into another, for example, inter- 

 changing one growing in a dry environment with another at home under wet conditions, and this 

 divergence in habitats gives opportunity for convergence in characteristics that "may sometimes 

 become practical identity." From these experiments they got results that led them to revise the 

 species of three genera as follows: 



Let us look a little more deeply into what these experimentalists have to teach us: Taxonomy 

 in biology, as the word indicates, is 



the science of arrangement or system, and hence classification [of the organic entities. Naming of species is but the 

 first step in taxonomy, and even their description is not the whole of this science, since taxonomy should mean the 

 placing of the species in nature's lines of descent; or, stated in another way, the placing of them so as to give] 

 the best possible record of evolution and relationship. . . Absolute adherence to phylogeny should be the basis of 

 classification. . . Evolution is the process and phylogeny the record of descent. 



Taxonomy leans most heavily upon morphology, and should bring to its aid not only histology 

 and physiology, but ecology and genetics as well. 



Indeed, if it [taxonomy] is to reflect evolution as accurately as it should, it must regard physiological adjust- 

 ment as the basic process, and morphological and histological adaptations as the measurable results. This 

 means that the taxonomist of the future will think in terms of evolutionary processes, and will learn to treat his 

 morphological criteria as dynamic rather than static. [Quantitative ecology] traces the evolution of new forms in 

 minute detail, in so far as they arise through adaptation or variation, and consequently furnishes the only direct 

 evidence of relationship by descent. It affords the sole method of testing the manifold assumptions of existing 

 taxonomy, and provides the foundation upon which an objective and permanent taxonomy may be reared. 



At present, however, taxonomy is more often the artificial expression of organic evolution; arti- 

 ficial because the science is as yet mainly one of personal opinions, and naturally this is more true in 

 paleontology than it is in neontology. 



A natural classification must maintain as well as reveal the different degrees of relationship as expres- 

 sions of different stages of evolution, and it can do this most accurately with genera, sections, species, and variads, 

 where the lines of evolution are still in a condition conducive to experimental study . . . Since the limits of orders 

 and families are determined by genera, and the limits of genera by species, the whole problem resolves itself into 

 a statistical and experimental study of species and their evolution (pp. 3-6). 



It is now common practice to regard the genus merely as a concept, and often as an artificial 

 one. Regarding this. Hall and Clements say: 



This is doubtless true for those who regard the genus merely as a pigeon-hole, chiefly convenient for the 

 filing of new species. Such a view has its justification in the usual practice of making genera. . . It is not sup- 



"The Phylogcnetic Method in Taxonomy. The North American Species of Artemisia, Chrysothamnus and Atriflex. 

 Carnegie Institution of Washington, Pub. No. 326. See also Clements, An Ecologic View of the Species Question. Amer. 

 Nat., vol. 42, 1908, pp. 253-264. 



