146 TEST CASES [ch. xiii 



If the conditions begin to change in any place, the new ones 

 may encourage some plants, and discourage others, so that 

 natural selection may in time effect a change of the local flora, 

 some plants coming in from other near-by regions where condi- 

 tions are more or less like those which now obtain in the locality 

 under consideration, and some of the local ones perhaps dying 

 out in that region. Possibty, even, under the stimulus of changed 

 conditions, new endemics may appear. But while plants that are 

 really very local may be completely killed out by a serious change 

 of climate or other conditions, it is very unlikely that this will 

 happen with plants that are already widely dispersed into a 

 considerable variety of conditions. To imagine that a species that 

 has become well adapted to certain conditions that occur in one 

 country has become thereby adapted to those that may occur in 

 some country widely separated from the first, is to press the idea 

 of adaptation altogether beyond possibility. 



TEST CASE XXVI. AGE AND AREA 



There is no need to add much to the description already given 

 in chap. iii. One of its striking features is the proof that it gives 

 that the distribution of a plant within a country, such as Ceylon 

 or New Zealand, goes on the average with its total distribution 

 outside that country. When one considers the differences in con- 

 ditions that must exist, this goes to show that natural selection, 

 in the sense of gradual structural adaptation, can have had little 

 or nothing to do with the distribution. What kind of an "adapta- 

 tion" can a species have acquired that enables it to go so far 

 afield, into so great a variety of conditions? And still more diffi- 

 cult is it to explain why the species that are endemic in any given 

 country are usually closely related to these species of large and 

 widely ranging genera. 



In Ceylon, for example, and the same can be said of other 

 places, the species that are most widely dispersed locally, on the 

 average, are those that range beyond the South Indian peninsula, 

 i.e. beyond a line drawn from Bombay to Calcutta. The next most 

 widely dispersed occur in Ceylon and in the peninsula only, 

 while the least dispersed are the local or endemic species that do 

 not occur outside Ceylon. All, of course, as pointed out in Age 

 and Area, must be taken in averages, as an endemic in an old 

 genus (in Ceylon) might be much older, and occupy more ground, 

 than a newly arrived "wide", even if the latter also ranged to 



