70 THE PLANT SPECIES 



higher plants, by contrast, cryptic species are not at all uncom- 

 mon; examples can be found in nearly every large family 

 and in a majority of large genera. When we recognize the 

 fact that cryptic species are a common byproduct of polyploidy, 

 and that polyploidy is a commonplace mode of speciation in 

 plants but an exceptional phenomenon in animals, we can readily 

 account for the observed differences between plants and animals 

 as to the frequency of sibling species. 



Similarly, asexual reproduction, which is extremely rare in the 

 vertebrates and uncommon in the insects, is a normal means of 

 propagation in higher plants. Vast polytypic assemblages of 

 asexual microspecies, such as the agamic complex in Crepis sect. 

 Psilochaenia or the clonal complex of the Opuntia phacacantha 

 group, have no known counterpart in the higher animals, al- 

 though they represent a standard pattern of evolution in plants. 



A prominent difference in the mode of reproduction between 

 higher plants and higher animals is the almost universal dioecism 

 of the latter and the very frequent hermaphroditism of the 

 former. Many hermaphroditic plants, moreover, reproduce par- 

 tially or predominantly by self-pollination. As already noted, 

 however, the species problem does not seem to be materially 

 increased in plants over that in animals by this difference in mode 

 of reproduction alone. Apparently a very low rate of natural cross 

 pollination in many autogamous plants is consistent with an in- 

 tegration of the members into species units. 



Natural hybridization seems to play a rather different role in 

 the evolution of plant and animal species. Natural hybrids have 

 of course been recorded in various animal groups, as exemplified 

 by Cockrum's (1952) check list for birds. Attention lias also been 

 paid to the effects of natural hybridization on the variation pat- 

 tern and on speciation in several groups of animals, as for in- 

 stance birds (Mayr and Gilliard, 1952; Sibley, 1954; Miller, 1955), 

 mammals (McCarley, 1954; Rudd, 1955), amphibia (Blair, 1941, 

 1955; Volpe, 1952), 'fishes (Hubbs, 1955), butterflies (Hovanitz, 

 1919), ;iud snails (Ilubendiek, 1951), to cite only a few repre- 

 sentative studies. In the fishes this hybridization may be a fairly 

 common and significant process. The consensus among zoologists, 



