Chapter IX — 139 — On Migration 



As a rule, however, species do not possess unlimited tolerance nor 

 the capacity to adapt themselves to changing conditions as rapidly as 

 these changes occur. This was pointed out by Darwin {I.e., p. 421), 

 who remarked that in nature species "probably change much more 

 slowly, and within the same country only a few change at the same 

 time. This slowness follows from all the inhabitants of the same 

 country being already so well adapted to each other, that new places 

 in the polity of nature do not occur until after long intervals, due to 

 the occurrence of physical changes of some kind, or through the immi- 

 gration of new forms. Moreover variations or individual differences of 

 the right nature, by which some of the inhabitants might be better 

 fitted to their new places under the altered circumstances, would not 

 always occur at once". 



Changes in generic characters proceed still more slowly. Imprints 

 (buried under the earth for hundreds of thousands of years) of many 

 fossil angiosperms of the Tertiary period, which to this day form part 

 of several present-day floras, closely resemble modern representatives 

 of the same genera. As Darwin (1868, Chap. 26, p. 352) put it, 

 "Generic characters are less variable than specific characters; and the 

 latter are those which have been modified by variation and natural 

 selection, since the period when all the species belonging to the same 

 genus branched off from a common progenitor, whilst generic charac- 

 ters are those which have remained unaltered from a much more re- 

 mote epoch, and accordingly are now less variable". 



It is well known that as a result of changes in habitat conditions 

 there may arise, by mutation, new races, enabling a species to expand 

 its area beyond the limits restricting the spread of the initial species, 

 but there is no case known of such mutational changes embracing en- 

 tire floras. They affect only separate species and are more or less 

 accidental in character. These considerations give grounds for pre- 

 suming that, as a rule, the evolution of the biological and physiological 

 characteristics of plants, involving adaptation to new habitat condi- 

 tions, cannot proceed as rapidly as changes in habitat conditions. 

 Consequently, in case of such changes, a plant must change its place 

 of abode or perish. It is able to change its place of abode, thanks to 

 various adaptations for the dissemination of its progeny. The pos- 

 session by a plant of the capacity to tolerate within certain limits 

 changes in habitat conditions serves the plant, when it finds itself 

 outside its "range of tolerance", as an incentive to change its habitat 

 in the direction of a return to environmental conditions suitable for it, 

 i.e., as an incentive to migration. Let us quote Good (I.e., pp. 157-9) : — 



"Taking into account the great number of plants species and the comparatively nar- 

 row world gamut of many external conditions it is not difficult to imagine that more than 

 one species may have similar ranges of tolerance, especially as regards some factors. This 

 is strictly in accordance with the observed facts of plant competition and is a satisfactory 

 explanation of it. It supposes that the tolerances of the two species, although similar in the 

 main, are actually very slightly different and that this difference is extremely important in 

 deciding the result of competition. This result may be a balanced deadlock, suggesting 

 that there is nothing to choose, in tolerance, between the species. More often competition 

 will result in the establishment or development of one species at the expense of the other, 

 and this is assumed to mean that, in some very subtle way, the tolerance of the victorious 

 species is more closely correlated with the actual conditions 



