76 THE PLANT SPECIES 



zation runs the full course to secondary speciation, the original 

 extreme populations undergo a progressive change in status from 

 species through semispecies to subspecies. 



Many existing plant groups, as exemplified by certain sections 

 of the willows, oaks, birches, columbines, Ceanothiis, and Gilia, 

 are aggregations of hybridizing semispecies. The major elements 

 composing these vast polymorphic assemblages are not good 

 species in the usual sense because they interbreed more or less 

 freely with one another. It is equally inappropriate to regard them 

 as merely races and the whole assemblage as a single species, 

 because the extreme forms show partial reproductive isolation 

 from one another and have attained a degree of morphological 

 differentiation equivalent to that of good species in other 

 branches of the phylad. The solution suggested for this difficulty 

 of the species concept is to recognize the whole assemblage as a 

 unit of interbreeding higher than the species and to designate it 

 by the term syngameon of Lotsy. The syngameon is defined as a 

 group of hybridizing species or semispecies. 



The "splitting of the species" is not without precedents in biol- 

 ogy. The conceptual difficulties of defining the individual in plant 

 groups which propagate asexually forced a recognition of two 

 classes of units: individual, as defined on the basis of physiolog- 

 ical autonomy, and clone, the sum total of the individuals de- 

 scended from a single zygote. Physiological geneticists are cur- 

 rently splitting the gene into two distinct units, having found that 

 the unit of biochemical activity in the chromosome is not the 

 same as the unit of crossing over. Similarly in higher plants it is 

 necessary to recognize the existence of both the species and the 

 syngameon or complex of species linked together by hybridiza- 

 tion. 



The final question as to why the syngameon has developed as 

 a common evolutionary unit in higher plants and not in higher 

 animals is tantamount to the question why plant species have 

 been so much more affected by hybridization than animal species. 

 A comparison of higher plants and animals with respect to the 

 degree of organization of the genotype may help to explain these 

 differences. The great complexity and internal coordination of 



