CH. viii] DIFFERENTIATION 69 



say, in Europe, was able to spread as far as New Zealand. It 

 could not become, in Europe, adapted to the conditions of New 

 Zealand, and its appearance there must be due simply to the 

 chance that the conditions resembled one another in both places, 

 and that there were conditions in between that were not dissimilar 

 at the time that the plant reached them. There can be no doubt 

 that as a plant moves very slowly about the world, it can become 

 adapted as it moves to the slightly different conditions that occur 

 at each move. If it come to a place where the change is too 

 sudden for it to adapt itself, it will then have come against a 

 barrier to further spread — an ecological or a climatic barrier, to 

 be added to the barriers of physical nature, such as high moun- 

 tains, or open seas, that so often occur. By the formation of one 

 of these barriers after the plant has passed, the distribution of a 

 species may become discontinuous. 



The success of a species under natural selection means usually 

 the greater or less extermination of the one from which it 

 descended, and which was not so well adapted. Under the theory 

 of differentiation, on the other hand, which goes with that of age 

 and area, the large and " successful " genera are simply the oldest, 

 while the small and local are in general the youngest. There is no 

 special adaptational reason for size or spread, so that, within any 

 close circle of relationship (which will more or less ensure the 

 same general reactions to the outside world), the rate of spread of 

 two or more forms will not usually be widely different. This is the 

 essence of the theory put forward in Age and Area. 



In diagram 7 we have indicated what we imagine to be the 

 probable general course of evolution under the theory of dif- 

 ferentiation. The family is represented at the start by a solitary 

 monospecific genus A, which will throw off new forms by muta- 

 tions. At first they will probably be produced very slowly indeed, 

 but as A increases its numbers (and with them its area, thus 

 probably coming under the stimulus of different conditions) will 

 probably appear more rapidly. Whether the earliest mutations 

 will be more often specific or generic we have no idea, but most 

 probably a second genus B will be "thrown" before so very long 

 a time. This will again begin slowly, and it will be a long time 

 before it throws a new genus Bb, whereas A will probably throw 

 its second new genus C before Bb appears. All this, of course, is 

 dealing in averages, and we do not know that this particular B 

 will necessarily be slower than A, though on the average the 

 second genus will be behind the first throughout. On the average 



