Chapter IX —141— On Migration 



This, as Good himself states, is "the most crucial point of the whole 

 theory", for, if both tolerance and climate changed at an equal rate, 

 there would be no plant movement and the entire theory would col- 

 lapse. But usually this does not occur, since evolutionary changes in a 

 species, as we pointed out above, proceed considerably more slowly 

 than changes in habitat conditions. The movements of floras known to 

 us sufiice to show that these changes are not simultaneous. They may 

 coincide in time and rate only in case there occurs a mutation affecting 

 the tolerance of the species. 



Quoting Good further (pp. 163-165): — 



"In the simple case described above there is a very great difiference between the indi- 

 viduals in the van of the movement and those in the rear. The correlation between climate 

 and area in the van is never seriously upset: there is simply a gradually unfolding area 

 into which dispersal will be effective. The conditions in the rear are quite different. Here 

 the potential area is continually diminishing and the possibility of successful dispersal is, 

 for many individuals, becoming increasingly small, so that the plants are constantly in 

 incomplete harmony with their external conditions. . . . 



"This hypothetical example of the working of tolerance is, as has been stated above, 

 the simplest imaginable and in nature the circumstances will almost always be more com- 

 plex. . . . 



" In the example given, only one species is supposed to be affected by climatic change, 

 but this is clearly an ideal condition. In all normal circumstances, climatic or other ex- 

 ternal change will affect a number of species, generally of varied types. It is, moreover, 

 highly improbable that the tolerances of all the species will be alike. They will most 

 likely vary greatly. The realisation of this leads to a most important conception in the 

 theory: its ability to exert a selective or sifting effect among species. 



"Suppose, as another purely hypothetical illustration, that the climatic change of 

 temperature already taken as an example affects one hundred species instead of only one. 

 Allowing for an average amount of variation in specific tolerance it is likely that the change 

 will react upon different species in different ways. Some it may affect directly and imme- 

 diately, but others it may affect either indirectly or not at all, their ranges being deter- 

 mined more particularly by their relations to factors other than temperature. This being 

 so, change of temperature will neither reduce their potential areas in one direction nor 

 increase it in others and they will remain unmoved. The total result of this will be that 

 the temperature change will cause movement in some species but not in others. In terms 

 of movement, some will advance in the direction of change and others will remam station- 

 ary so that there is a selective effect 



"In other directions, too, the original simple scheme must be augmented if it is to 

 give anything like a complete picture of the effects of the theory. For e,xample a climatic 

 change will rarely be confined to a hmited region. A general lessening of temperature, like 

 that imagined, would in all probability extend over at least a large part of a hemisphere 

 and even perhaps over the whole world, with the result that there would be a general 

 movement towards the equator of parallel temperature zones. In accordance with the 

 postulated effect of tolerance, this means that one geographical zone will sooner or later 

 come to possess the flora originally characteristic of an adjoining zone. The area rendered 

 unsuitable for one set of species will become in turn the potential area of another set. 

 This, combined with the sifting effect just described, gives a picture, not only of floral 

 migration in bulk, but also of the intermingling of diverse floral elements. Consider the 

 floras of two adjacent parallel climatic regions affected by, let us say, temperature change. 

 Since presumably only a portion of the species in each zone (the proportion may be very 

 high) is susceptible to this particular external change, some species will move under its 

 influence and others will not. But a similar selection is at work in both zones and the 

 result will be that the species left behind by the movement of the zone in advance will 

 become mixed with the species which have advanced with the movement of the zone in 

 rear". 



Moreover, it should be kept in mind that a climatic change, usually 

 involving changes not in one but in several factors, induces changes in 

 all the habitat conditions {i.e., in all the ecological conditions affecting 



