Chapter IV 



THE ORIGIN OF AREAS 



Having acquainted ourselves with that center of an area, primary 

 or secondary, from which a genus or a species started its dispersal 

 resulting in the formation of its area, we may now pass to an exami- 

 nation of the still broader problem of the origin of the area itself. The 

 diversity of areas of plants now inhabiting the globe and the entirely 

 different character of the areas of many of these plants in former 

 geological periods, as testified to by the location of their fossil remains, 

 indicate that the origin of areas of genera and species differing in age, 

 in ecological and biological type, and in adaptabihty to conditions of 

 habitat and dispersal cannot all be explained in the same fashion. 

 Such a mechanistic approach to this difficult problem would not in the 

 least conform to the great diversity existing in the vegetable kingdom. 



However, in order to present the problem of the origin of an area 

 in all its entirety would mean to present the problem of species-forma- 

 tion in nature in all its complexity and diversity. If this were here 

 attempted, this chapter devoted to the origin of areas would exceed in 

 length the limit set for the entire textbook. Hence, it is quite evident 

 that a full exposition of the problem of species-formation cannot find 

 place in the present volume. We can merely give a very brief survey 

 of the present status of this problem. And, first of all, we wish to 

 emphasize the fact, proven beyond any doubt by modern science, that 

 species arise in nature in various ways. Recent advances in biology 

 show that the diversity of species cannot be ascribed solely to a gradual 

 intensification of characters. 



Mutations — particularly the appearance of new forms as a result of 

 autopolyploid doubling of the chromosome number induced by the 

 action of external factors— and, to a less extent, hybridization, accorn- 

 panied by allopolyploid changes in chromosome number, played in 

 former geological periods and still play at the present time a prominent 

 role in species-formation. This does not mean that in these processes 

 of species-formation natural selection takes no part. On the contrary, 

 it retains all its significance, determining the survival of the fittest and 

 their further evolution. 



According to Darwin, a species arises in one definite place on the 

 globe. From this place— thanks to a natural tendency to expand its 

 territory, manifested, for instance, in the abundance of seeds or spores 

 formed and in numerous adaptations of fruits and seeds enabling the 

 future progeny to gain a foothold as far as possible from the mother 

 plant — it begins its dispersal. It continues to expand its area in all 

 possible directions until it encounters geomorphological or ecological 

 barriers that it cannot cross because of purely mechanical or biological 

 reasons. Hence, it follows: first, that the older a species, the larger 

 the area it occupies, provided, of course, that the above-indicated 

 barriers do not at the very outset restrict its dispersal or that subse- 



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