V. GRANT 69 



of influence in plants and animals. A comparative approach to 

 the species problem may thus throw some light on its causes in 

 higher plants. Such an approach may even shed light on the 

 ultimate causes behind the immediate causes. 



Since the factor of gradual speciation is common to both ani- 

 mals and plants, it cannot be cited as a cause of the proportion- 

 ately greater species problem in plants unless certain additional 

 assumptions are made. If the rate of speciation is higher in plants 

 than in animals, and if this speciation is still continuing in plants 

 but has tapered off or ceased in many animal groups, the propor- 

 tion of poorly defined species due to incomplete divergence will 

 be greater in plants. 



The first assumption can be safely granted. There are about 

 six times as many species of angiosperms as vertebrates in the 

 world (ca. 250,000 to 40,000), even though the angiosperms orig- 

 inated later in geological history than the vertebrates. Whether 

 this speciation is continuing into the recent period more actively 

 in plants than in animals, however, is difficult to judge with the 

 evidence available. The Age of Man has probably retarded evo- 

 lution in the larger land mammals and speeded evolution up in 

 weedy and cultivated plants, but the contrast between higher 

 plants and animals with respect to the clarity of species is not 

 confined to these particular groups. All that we can safely con- 

 clude is that the process of gradual species formation will result 

 in a larger number, but not necessarily a higher proportion, of 

 poorly defined species in higher plants as compared with verte- 

 brate animals. 



Since, so far as we can judge, incomplete speciation is not a 

 major contributing factor to the species problem in plants, any 

 differences that do exist between plants and animals with regard 

 to the relative frequency of incompletely formed species can 

 probably be ignored for the purpose of the present analysis. 



A second cause of the species problem, the occurrence of sib- 

 ling species, does show marked differences in frequency in plants 

 and animals. Sibling species are of course well known in animals, 

 being fairly common in the Diptera and occurring also in the 

 vertebrates. In the latter they are apparently not common. In the 



