6] FOUNDATIONS OF MODERN DISTRIBUTIONS 175 



postglacial history or origin, such considerations again being 

 important as a basis of present-day distribution. So here again we 

 see instances of the significance of historical causes for an under- 

 standing of the present distribution of species and of their groupings 

 in vegetation. 



The Genetical Heritage 



The remaining aspect to be considered as part of the historical 

 background of plant distributions is the genetico-evolutionary one, 

 an outstanding example being afforded by polyploids, treated in the 

 next section. It seems reasonable to suppose that the physiological 

 tendencies and habitat preferences of particular plant entities have 

 long been much as they are today, as have, doubtless, many of the 

 habitats themselves, and that morphological (that is, of general 

 form) and anatomical (of internal structure) indications of ecological 

 relationships that hold nowadays are also largely applicable to plants 

 which lived in earlier ages. Indeed such assumptions are behind 

 many of our contentions regarding climatic and other changes in 

 bygone ages. Nevertheless, evolution has doubtless proceeded at 

 \arious speeds and with varying results throughout the period in 

 which there have been advanced forms of life on earth, and among 

 the characters affected have surely been such ones as migrational 

 abilities, acclimatization potentialities, and habitat preferences. 

 Consequently, we should consider in broad outline the evolutionary 

 tendencies that manifest themselves in these characters, and some 

 facts and fallacies of resultant areal indication, before proceeding 

 with the more practical parts of this treatise. 



Just as most obvious manifestations of form are inherited by each 

 generation from the last, and this process is repeated virtually ad 

 infinitum, so are different functions, different physiological attributes, 

 usually so inherited — including those which control the migrational 

 abilities, acclimatization potentialities, and habitat preferences of 

 different plants. Indeed these last three groups of factors are best 

 considered as one, being in any case all dependent on inherent 

 tendencies that can scarcely be separated. An outcome of this 

 inheritance, generation after generation, in particular lines or strains 

 of plants, is the obvious suitability of certain plants for certain areas, 

 the converse holding often more strongly — namely, that certain 

 other plants are unfitted for growth in these areas. Just as evolution 

 of form results from the action and often interaction of one or more 



