GEOGRAPHICAL DISTRIBUTION 393 



such cases it must be supposed that the ordinary agencies of 

 dispersal have not sufficed to bring the species to all regions in 

 which it can thrive successfully. 



The natural barriers to the spread of plants are oceans, 

 mountain ranges, deserts, etc., and it is significant that isolated 

 islands (e.g. New Zealand) are peculiarly the home of species, 

 so-called endemics, which are found there and there alone. There 

 is, moreover, good reason for believing that, in a region devoid 

 of natural barriers, the area over which a species is distributed 

 is proportional to the age of the species or the time that has 

 elapsed since its introduction. The capacity of a species for 

 extending its geographical range, when the new territory is 

 attained, depends on the efficiency of its seed-dispersal mechanism 

 and the rapidity of its spread by this or by vegetative means. 



The theory of evolution explains the resemblances between 

 the members of a genus or family as the necessary consequence 

 of their origin from a common ancestor or from closely related 

 forms. It has already been seen how new types can arise as a 

 result of mutation or hybridisation. Since these show a great 

 resemblance to their known parents, it is reasonable to regard 

 the many features in common, between species of a genus or 

 between the genera of a family, as indications of a natural affinity 

 between them. What has already been said, then, with regard 

 to the distribution of species should also apply in a general way 

 to genera and families, if these really comprise groups of forms 

 with natural affinities. In many cases, indeed, the same prin- 

 ciples are applicable ; for example, the genus Commidendron (a 

 member of the Compositae), with three species, is restricted to 

 St. Helena, and whole families are sometimes largely confined 

 to definite areas, as the Epacridacese (which are closely allied to 

 the Heather-family) to Australia and Tasmania (cf. also Fig. 228). 



Geological research has shown that oceans and continents 

 have undergone manifold changes, even during the period of 

 existence of many living species, and these secular changes 

 probably afford the clue to the discontinuous distribution of many 

 species and families. For certain groups, now represented only 

 in widely separated areas over the earth's surface (e.g. the Cycads), 

 are known to be ancient, and may well have attained their wide 

 distribution before the present barriers were as pronounced or 



