SPECIES IN THE MAKING 109 



wonderful power of dispersal, and have become cosmo- 

 politan. Moreover, we find that some species are 

 numerically very abundant, others very rare ; that rare 

 and abundant species have often invaded each other's 

 territory, and exist side by side. 



Whilst we often find a number of species, fifty or more, 

 so much alike that we unite them in a single genus (as, 

 for instance, in the case of the cats, lions, tigers, leopards, 

 which form the genus " Felis," and the hundred or more 

 species of the hedge brambles or blackberries, which form 

 the genus " Rubus "), there are many species which to-day 

 have, as it were, lost all their relatives and stand alone, the 

 solitary species in a well-marked genus, or have perhaps 

 only one other living co-species. And sometimes (curiously 

 enough) that one co-species is an inhabitant of a region 

 very remote from that inhabited by the other. Thus the 

 two living mammals called tapirs (genus Tapirus) inhabit, 

 the one the Malay region, and the other Central America. 

 This is explained by the fact that tapirs formerly existed 

 all over the land-surfaces of North Europe, North Asia, 

 and North America, which connect these widely-separate 

 spots. We find the bones and teeth of the extinct tapirs 

 embedded in the Tertiary deposits of the connecting 

 regions. 



Once we have gained the fundamental conceptions 

 as to what is meant by a " species," we are able 

 intelligently- to consider innumerable facts of the most 

 diverse kind as to their peculiar structure and colours, 

 their number, localities, their interaction and dependence 

 on other living things, their modifications for special 

 modes of life, their isolation or their ubiquity. We can 

 discuss their genetic relations to one another, and to 

 extinct fossil species, which have all been to a very large 

 extent " accounted for " or " explained " by Mr. Darwin's 

 theory of the origin of species by the natural selection 



