192 THE MONOTYPIC GENERA [pt. ii 



about halfway down New Zealand, while another begins there 

 and reaches the remainder of the distance? Further, these mono- 

 typic endemics have an average range of about 446 miles, and 

 in a varied country it is a little difficult to imagine conditions to 

 which they can be just suited in such a range. 



If one take the families in groups of ten, in order of their size 

 in the world (as measured by the number of genera given in my 

 Dictionary), one finds that the column of monotype numbers 

 follows that of numbers of genera with wonderful closeness; the 

 first exception comes only at the seventeenth group of ten 

 families, a group including only 59 genera, or six per famil5^ 

 Even beyond this the numbers continue closely parallel, and 

 there is only once an exception. The percentages also show clearly 

 that (just as with endemics) the greatest proportion of mono- 

 types is in the largest [i.e. on our hypotheses, in general the 

 oldest) families, falling steadily from 40 per cent, in the first 

 forty families to 30 per cent, in the final group of 131 very small 

 ones. 



Analysing from my Dictionary, as corrected to date, the pro- 

 portion of monotypes in the various families, one finds that in 

 the families with over 100 genera the percentages vary between 

 28 and 56, With three-quarters of the M'hole total between 33 

 and 44. Those below 100 genera vary betAveen 11 and 68 per 

 cent., or tAvice as much, with three-quarters betAveen 23 and 50. 

 The percentage in the larger families is evidently a little higher, 

 as has already been pointed out. 



There is a fair amount of difference, therefore, between indi- 

 vidual families. In the first ten, the largest percentage is in the 

 Asclepiads (54 per cent.), the loAvest in the Orchids (35 per cent.), 

 but there is not the least reason to suppose the former to be a 

 specially moribund family. Other families Avith more than 50 per 

 cent, of monotypes are Burseraceae, Lythraceae, Menisperm- 

 aceae, Portulacaceae, Saxifragaceae, Juncaceae, etc. 



Explanations of the facts of monotypism have followed much 

 the same lines as those of endemism, the genera being regarded 

 as local adaptations or as relics, according to taste. But Avhat 

 has been pointed out above shoAvs that there is a very definite 

 arithmetical relationship betAveen monotypes and genera of 

 larger size, not only on the total, but also in very fair detail. 

 This alone is almost a conclusive argument against either of the 

 suppositions just mentioned as a general explanation, though 

 of course there must be many individual exceptions, better 



