THE EXTINCTION OF SPECIES. 901 



The most striking examples of genera that have become extinct are afforded by 

 the alliances to which the Club-moss and the Horse-tail respectively belong (see pp. 

 713 and 716). Those genera most exposed to the danger of extinction are such 

 as are only represented by a single species (termed monotypic genera). This risk 

 is intensified if the species in question only occurs in one district, as is the case, 

 for instance, with Welwitschia. The genus Rhodotharanus of which only one 

 species, R. Chamcecistus, lives at the present day, and the genus Azalea which is, so 

 far as we know, represented by the species A. prociimbens (Loiseleuria procuvibens) 

 alone should, on the other hand, have a better chance of escaping extinction. For 

 the area of distribution of the former is broken into two sub-areas by the broad 

 stretch of land reaching from the Eastern Alps to the Altai Mountains, whilst the 

 Azalea grows not only on the high mountains of Central and Southern Europe but 

 also in the Arctic regions, leaving a great space between the two areas uninhabited 

 by its kind. Thus, presumably, even if such a species were to vanish entirely from 

 one of its sub-areas of distribution in consequence of changes in climatic conditions 

 it would still be represented by individuals growing at some spot which, being 

 remote from that sub-area, would in all probability not have been affected by the 

 alterations in question. 



The number of species that have died out is extremely large. Every gi-oup of 

 species which comprises both living and fossil members affords instructive examples 

 in this connection. It is assumed that of species now living the endemic kinds have 

 their existence most seriously menaced, or, in other words, are exposed to the most 

 speedy extinction. If the restricted areas of the South-eastern Alps, where Wul- 

 fenia Carinthiaca, the most famous of the endemic species, is native, were to 

 undergo a change of climate which no longer permitted the propagation of that 

 species either by sexual or asexual methods, and at the same time rendered its 

 migration impossible, it would be only a matter of time before Wtdfenia Carinthiaca 

 disappeared completely from the face of the earth. The genus Wulfenia would not, 

 it is true, thereby become extinct, for a second species named Wulfenia Amherstia 

 occurs in the Himalaya. But as this species, too, is endemic it might easily be 

 overtaken by the same fate, and then the entire genus would have died out. It is, 

 however, conceivable that the change of climate supposed to affect the South-eastern 

 Alps, instead of causing the extinction of Wulfenia Carinthiaca, might result in the 

 expansion of its area of distribution, and that no such hindrances to its migration 

 as at present prevail should exist. The two species now inhabiting such widely 

 remote districts might then conceivably come together and cross with one another, 

 with the result that new species would be produced in the genus Wulfenia. It will 

 be seen from these examples that one cannot be too cautious in dealing with assump- 

 tions concerning the future destinies of species. Many endemic species are probably 

 doomed to extinction in the near future; but it is also not imjjossible that they may 

 instead be called upon to play an important part in days to come. 



An account has already been given (pp. 590, 592) of the manner in which 

 nature affords compensation for the extinction of species, and how new species, the 



