96 TEST CASES [ch. x 



decreasing size, but increasing numbers, with decreasing age. The 

 result would be to give one of the hollow curves which we have 

 described above. A little thought will soon show that the diminu- 

 tion in size will not be proportionate to that in age, for the older 

 that a genus is the more rapidly will it tend to gain upon those 

 younger than itself (66, p. 34), 



As a genus or species (they are the same at the start) increases 

 in number of individuals and in area occupied, it w411 begin to 

 "throw" offspring differing from itself, by mutations occurring 

 at infrequent intervals, sometimes of generic rank, but more often 

 of specific. The average size of a genus is about fourteen to fifteen 

 species, but this does not mean, as one is tempted to suppose, that 

 a generic mutation may occur once in fourteen to fifteen times. 

 Rather it means that the average age of a genus may be more 

 or less represented by the average age of those which possess 

 fourteen to fifteen species. Some of the throws will be undoubted 

 species, some undoubted genera, some again of doubtful rank. 



Supposing, which seems the most probable, that a new species 

 or genus begins upon a small area, it will probably be a very long 

 time before it occupies a more considerable space with more 

 individual representatives. But while it may wait a very long 

 time for the first throw, it would seem probable that the frequency 

 of the throws will on the whole increase w4th the number of the 

 individuals in the species, which in turn will tend to increase more 

 and more rapidly as time goes on (cf. Age and Area, pp. 33-4). 

 The first line of descent, that from the original genus (and species, 

 of course) of the family, will always have the start of the second, 

 which arises from the first generic throw of the original genus. 

 But as time goes on, there will be a continually increasing number 

 of lines of descent with the continual formation of more and more 

 genera to head them, so that at last we shall get the familiar curve 

 shown by any table of numbers of species in the genera of any 

 particular family of reasonable size. Thus a recent enumeration 

 of the Caryophyllacea^ (35) gives the following figures (bigger 

 genera obviously rounded to nearest ten or more) : 



400, 300, 160, 100, 100, 90, 70, 40, 40, 30, 30, 30, 25, 23, 20, 

 20, 20, 20, 20, 18, 16, 15, 12, 10, 10, 10, 8, 7, 6, 6, 6, 5, 5, 5, 

 5, 4, 4, 4, 4, 4, 3, 3, 2, 2, 2, 2, 2, 2, 2, 1, 1, 1, 1, 1, 1, 1, 1, 1, 

 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1. 



If, then, genera are formed upon this principle — and that this 

 is quite a probable approximation to what really happens is 



