68 THE PLANT SPECIES 



grammatic illustration of the changing composition of a syn- 

 gameon within a homogamic complex is given in Fig. 3. 



Reference to the fertility or sterility of the hybrid types is pur- 

 posely omitted from the foregoing definition, contrary to earlier 

 efforts in the era of Lotsy, Turesson, Danser, etc., as not being 

 of major importance for the development of a syngameon. 



In a syngameon the original biological species either may per- 

 sist as discrete entities, as exemplified by Gilia capitata and G. 

 millefoliata in the syngameon to which they belong, or come to 

 occupy a definitely subordinate position to the total complex. In 

 the latter case the species as a biological unit is found in varying 

 degrees of dissolution. When species-like entities are discernible 

 on the basis of partial reproductive isolation externally contrast- 

 ing with free gene exchange internally, these constituent units 

 can be designated, following Sibley ( 1954 ) , as semispecies. ( This 

 term was used earlier by Mayr (1940) in a somewhat different 

 sense.) The syngameon of Gilia tenuiflora-latiflora, for example, 

 consists of four semispecies as shown in Fig. 2. In some syn- 

 gameons, finally, the original species have become more or less 

 completely swamped by hybridization, as has happened with 

 staminea and capitata in the development of the present Gilia 

 capitata. 



The species, semispecies, or remnants of species are organized 

 into a higher unit by frequent or infrequent gene exchange across 

 presently or formerly existing reproductive barriers. The most 

 inclusive interbreeding population is, consequently, not the spe- 

 cies but the syngameon. Depending on the degree of integration 

 attained in a syngameon, its constituent species may be preserved 

 as such or partially or completely lost. The syngameon is in any 

 case a biological unit in its own right. 



The Botanical versus the Zoological Species Problem 



If comparisons are made between taxonomically well studied 

 groups of seed plants and vertebrate animals, the boundaries of 

 species are found to be, on the average, less well marked in the 

 plants than in the animals. The causal factors of the species prob- 

 lem can accordingly be expected to exhibit a contrasting degree 



