CH. XII] SIZE AND SPACE 115 



number of species representing that genus in that country, or, 

 in other words, that the principle of Size and Space is vaHd. 

 Very httle consideration, however, is required to show that in 

 general a genus of many species occupies a larger area than an 

 allied genus of few species. It is not perhaps always realised 

 how close the agreement really is, when one considers a number 

 of allied genera (as with Age and Area) between the size of a 

 genus (as marked by the number of its species) and space occu- 

 pied. Everyone knows that Senecio or Astragalus, with 1500 or 

 more species, occupies an enormous area, whilst monotypic 

 genera like Fatsia (Japan), or Welwitschia (south-west Africa), 

 or, again, like lonopsidium (Portugal) or Kitaihelia (Lower 

 Danube) occupy small ones, and genera with intermediate 

 numbers of species often occupy areas between these extremes. 

 But, on the other hand, people point^ to such a genus as Hijo- 

 puris, with one nearly cosmopolitan species, or Veronica, with 

 about 80 species in New Zealand, and maintain that there is no 

 connection between size and space. Now there is no doubt that 

 these exceptions to the rule are very numerous and very im- 

 portant, so that it would be in the highest degree dangerous to 

 draw a rule with limits as narrow as those for Age and Area 

 (ten allied species); but we are, nevertheless, of the opinion that 

 such a rule may be drawn, in such a form, say, as " Within any 

 circle of plants of near affinitv, living under similar ecological 

 conditions, the areas occupied, taking the genera in groups of 

 ten, will vary with the number of species in the genus, being 

 large when that is large." It is to be noted that proportionate 

 areas are not claimed; one would probably have to deal with 

 the genera by hundreds rather than tens for this. 



The number of species in a genus seems to bear a distinct 

 general relation to the variety of conditions that exists in its 

 range: for example, water plants in general ha\-e much fewer 

 species than land plants that cover the same area. It is clear, 

 however, that this is not a complete explanation, for Veronica 

 in the compa^ati^^ely uniform conditions of New Zealand, or 

 Eugenia or Sirohilanthes in those of Ceylon, is represented by 

 iiKiu}^ species, while some species arc able to stand a variety of 

 conditions, such, for example, as Cissampelos Pareira or Senecio 

 vulgaris. On the whole, however, greater variety of conditions 



^ "There is no necessary relation between the area a genus covers, and 

 the number of species it contains, though speaking generall}% monotypes 

 have a restricted area" (51, p. xxx). 



