E. V. Wulff -^4— Historical Plant Geography 



ing of the oldest species, those most widely distributed in New Zea- 

 land. To quote Willis (1922, p. 75): "In fact it was found that on 

 the average its species ranged nearly 300 miles more in New Zealand 

 than did those that did not reach the islands". 



As regards the way in which a country is peopled by invasions of 

 plants, Willis gives the following rule based on his general h3^othesis 

 {ibid., p. 83): "If a species enter the country and give rise casually to 

 new (endemic) species, then, if the country be divided into equal zones, 

 it will generally occur that the endemic species occupy the zones in 

 numbers increasing from the outer margins to some point near the 

 centre at which the parent entered". Applying this to New Zealand, 

 he found that all the genera in its flora adhered to this rule. 



Supplementary to his central "Age and Area" hypothesis, Willis 

 proposes a second principle called by him "Size and Space", which he 

 formulates as follows {ibid., p. 118): "If species spread in a country 

 mainly in accordance with their age, then it is clear that on the aver- 

 age some of those in the genera represented by most species will have 

 arrived before the first of those in the genera represented by few; ... on 

 the whole, keeping to the same circle of affinity, a group of large genera 

 will occupy more space than a group of small. The space occupied will 

 vary more or less with the number of species". 



From the foregoing follows also the final implication of Willis's 

 theory, viz., that monotypic genera, that is, genera with one species 

 only, like endemic genera and species, are "young beginners" that 

 have just commenced their geographical distribution. Here, however, 

 one must make the reservation (which Willis himself fails to make) 

 that this conclusion is not applicable to genera that have acquired 

 their monotypic character as a result of the dying out of most of their 

 species nor to ancient endemic genera and species. 



All of Willis's views may be summarized in this basic hypothesis — 

 the area of a species is proportional to its age. If this proposition 

 could be universally applied, it would simplify the solution of many 

 problems of botanical geography. But, as his critics pointed out, this 

 proposition is applicable only to certain genera and species, and so it 

 cannot serve as a general rule for determining the age of an area. For 

 instance, paleobotanic data show that some genera now occupying 

 small areas were widely distributed in the past and are often older 

 than genera now having extensive areas. The same applies to those 

 endemic species that are the descendants of species at one time widely 

 distributed but whose areas were much contracted (Berry, 1917). 

 In his later papers and in his book Willis, in answer to the deluge of 

 criticisms, reformulated his Age and Area hypothesis, qualifying it by 

 so many reservations that it became very complicated and practically 

 unworkable. In its latest version it read: "The area occupied at any 

 given time, in any given country, by any group of allied species at 

 least ten in number, depends chiefly, so long as conditions remain 

 reasonably constant, upon the ages of the species of that group in that 

 country, but may be enormously modified by the presence of barriers 

 such as seas, rivers, mountains, changes of climate from one region to 

 the next, or other ecological boundaries, and the like, also by the 

 action of man, and by other causes" (Willis, 1922, p. 63). 



