V. GRANT 73 



a certain amount of transgression of species barriers in plants, 

 whereas in animals a more rigid isolation of species is of selective 

 value. This hypothesis is consistent with other underlying differ- 

 ences between plants and animals. 



The most basic difference between plants and animals is in the 

 degree of complexity of the individual organism. The develop- 

 ment of the relatively simple plant body is commensurate with 

 an open system of growth by which individual parts are built up 

 in series. The animal body is a far more complex and delicately 

 balanced system which must develop as a whole without dis- 

 ruption of the internal organization. 



Although very little is actually known about this question, it 

 is probable that the genetic determinants of growth and develop- 

 ment correspond in complexity to the degree of integration of 

 the individual organism. Thus the factorial basis of individual 

 character differences between races and species of plants is not 

 infinitely complex but can often be resolved into a fairly small 

 number of genes (Clausen, 1951; Baker, 1951; Burton, 1951; 

 Grant, 1950). The more simple characters in animals, such as 

 coat color or blood types in mammals (Castle, 1954; Race, 1950), 

 can likewise be explained in terms of a few genes. By contrast, 

 such fundamental characters of animals as the sex differences are 

 determined by exceedingly complex systems of genes which defy 

 an exact factorial analysis. 



The genie complement of any species is the product of a long 

 period of natural selection. The injurious effects of an unrestricted 

 influx of foreign genes from other species are the probable basis 

 of ad hoc isolating mechanisms (Dobzhansky, 1951). A species 

 possessing a highly integrated and finely balanced gene system 

 will, moreover, suffer relatively worse effects from hybridization 

 than one with a simpler and more loosely integrated set of ge- 

 netic determinants. In the case of the latter type of species, the 

 disadvantageous effects of interspecific gene exchange may even 

 be outweighed by the advantages resulting from an increase in 

 variability. 



The ability of a species to tolerate the presence of foreign genes 

 is thus a function of the degree of complexity and integration 



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