ARE ANY SPECIES UNIFORM? 



Or Should the Assumption of "Pure" Species be Discarded and Diversity 

 Recognized as the Normal Evolutionary Condition? 



O. F. Cook 



U. S. Department of A'j^riciiltiire, Washiugtou, D. C. 



AN ADDRESS by Professor Brad- 

 ley Moore Da\is on "Species, 

 Pure and Impure," published in 

 Science for Februar^^ 3, 1922, presents 

 the case of "impure" species very 

 effectively, while the "pure" species 

 are treated only by definitions that 

 lead, as Professor Davis says, to "what 

 is almost an abstraction." That so 

 little should be said of the "pure" 

 species appears the more significant if 

 we consider that species are not merely 

 definitions or abstract assumptions, but 

 natural groups of organic individuals, 

 affording the veritable subject-matter 

 of the biological sciences. 



Species are maintained by processes 

 of sexual reproduction, with continual 

 crossing of the individual lines of 

 descent, so that each species forms a 

 network of lines of descent. The ques- 

 tion of uniformity bears upon the 

 nature of the specific network. Are 

 there reasons for assuming that mem- 

 bers of the same species are uniform, 

 identical or homozygous? Or should 

 we think of the members of species 

 as normally diverse, with multifarious 

 germinal constitutions represented 

 among the difi"erent individuals and 

 lines of descent that are woven to- 

 gether? 



THE NETWORK OF DESCENT 



The network of descent is not to be 

 dismissed as an abstraction or mere 

 figure of speech. Though more diffi- 

 cult to describe or to define by arbi- 

 trary standards, the species as a whole 

 is not less real than the individual 

 "lines" of descent that are followed in 

 genetic or genealogical studies. It is 

 usual to deal with separately propa- 

 gated lines in the study of inheritance 



of particular characters, or for agricul- 

 tural purposes, but an individual line 

 of descent does not constitute a species. 

 We get only partial views of heredity 

 and evolution if the coherent, reticular 

 constitution of species, the normal 

 condition of crossing and weaving 

 together of the different individual lines 

 of descent, is left out of accoimt. Our 

 understanding of species governs in- 

 evitably our interpretation of the 

 nature and causes of evolution. To 

 think clearly about evolution is essen- 

 tial to constructive investigation, as 

 well as to the development of prac- 

 tical applications of biology. 



There would seem to be no doubt of 

 the existence of diverse, heterozygous, 

 or "impure," species, as Professor 

 Davis calls them. The doubt is whether 

 there are any "pure," uniform, species, 

 or any tangible reasons for maintaining 

 the assumption of uniform species, if 

 not supported by facts. Examples of 

 identical germinal constitutions are 

 found in twins and in plants propagated 

 by cuttings or by self-fertilization, but 

 these do not constitute species. Some 

 investigators have believed in "pure 

 lines" that would remain constant, 

 but "mutations" continue to appear in 

 self-fertilized or line-bred stocks. Even 

 with vegetative propagation, definite 

 differences arise through the "bud 

 mutations" that are now known to 

 occur in many plants. Mutation in 

 parthenogenetic generations of aphids 

 has also been announced recently.^ 



For purposes of description, species 

 are supposed to be uniform, but taxo- 

 nomic ideals of uniformity, however 

 useful in the study of classification, 

 should not be allowed to bring confu- 

 sion into evolutionary ideas. The ob- 



1 Baker, A. C. Journ. Washington Academy of Sciences, 12:320. 



