74 THE PLANT SPECIES 



attained by its system of genie determinants. The chances of ob- 

 taining a favorable recombination of the genotypes of two species 

 will vary from nil to high, depending upon the amount of internal 

 coordination in each specific genotype. A perfectly functioning 

 bicycle can be built up out of the parts of several different makes 

 of bicycle, but a watch containing the cogwheels of two or more 

 different makes will probably not run accurately and may not 

 run at all. 



It is highly unlikely that the partial to complete breakdown of 

 the species as a biological unit could continue on a wide scale 

 in many plant genera without the sanction of natural selection. 

 In order to explain this result we must show first that the relaxa- 

 tion of isolating barriers is not necessarily disadvantageous to a 

 population, and secondly that the resulting gene flow may confer 

 positive advantages. The hypothesis that a simple physiological- 

 morphological organization in plants, reflecting a correspondingly 

 simple genie pattern, permits a certain freedom in the exchange 

 of genes between species which is not enjoyed by the vastly more 

 complex animals, satisfies the first condition. The increase in 

 variability resulting from the pooling of the mutations of not one, 

 but two or several species populations is the advantageous aspect 

 of hybridization demanded by the second condition. Together 

 these conditions are capable of explaining an important differ- 

 ence between plants and animals as to the nature and behavior 

 of their species. 



Conclusions 



According to tin 1 biological species concept the species is a 

 population set apart from the rest of the living world by repro- 

 ductive isolating mechanisms. Its boundaries, therefore, should 

 be marked by a prominent gap in the variation pattern. Good 

 discrete species are in fact found in all major plant groups. These 

 well-defined species, however, constitute only a fraction of the 

 populations in their respective phylads. The botanist finds prob- 

 lematical species intermixed in varying proportions with good 

 species in most of the families and genera. This is in marked eon- 



