354 
Willis .— The Sources and Distribution of the 
The Relative Age of Trees, Shrubs, and Herbs, and the 
Sources of the New Zealand Flora. 
Relative age is a very large question indeed, and I may be pardoned 
if I point out that for the present, at any rate, age and area is quite incom¬ 
petent to solve it, though Prof. Sinnott so far extends the application of my 
hypothesis as to include that point among its possibilities. I have nowhere 
committed myself, so far as I can find, to an expression of opinion upon this 
question. These groups are ecological, not systematic, and nothing is more 
clear than that they are extremely polyphyletic, a fact which makes deter¬ 
mination of relative age difficult. There are very few families composed 
entirely of trees or herbs, and even among genera there are many contain¬ 
ing two or more forms. In Ceylon 87 show this, in New Zealand 12 : these 
include such welbknown genera as Polygala , Hypericum (in both countries), 
Abutilon , Hibiscus (in both), Helichrysum , Senecio (in both), Draco- 
phyllum , Solanum (in both), Euphorbia , Pkyllanthus, Ficus , Urtica , and 
many more. Even in such a markedly herbaceous family as Cyperaceae 
there is one tree, occurring in West Africa. Does Dr. Sinnott regard this as 
a solitary prehistoric Cyperacea now dying out ? 
Except in the case of genera, whose members are systematically allied, 
and where ( 17 , p. 337) it appeared to me probable that their distribution 
■ in wheels within wheels ’, exactly like the species in any one country, was 
easily explicable on the hypothesis of age and area, I have only applied 
this hypothesis to groups of twenty aided forms within a given country. It 
might perhaps be possible to prove, by careful application of the hypothesis, 
that within New Zealand trees were older than herbs, but that would prove 
nothing as to the absolute age of either group, except that the trees were 
the first comers. Only if it could be shown that they were the first comers to 
most countries would it be possible to say that they were in reality the 
older group. 
I have nowhere committed myself, so far as I can find, to the view that 
all plants occupying equal areas are of the same age, though I maintain that 
all are governed alike by the law of age and area. I have not attempted to 
decide, for instance, which of two genera or species, one a tree, the other 
a herb, is the older, when both occupy the same area in the same country. 
But in any case my figures show that each group is ruled by age and area, 
and a Dipterocarp (tree) with a radius of 200 miles in all probability bears 
the same relationship in age to one with a radius of 100 miles as does 
a Composite (herb) with a radius of 200 to one with one of 100. A priori 
it would seem probable that the herb would spread the more rapidly, but 
one must remember another complication, that some herbs are of open 
