196 Willis.— The Relative Age of Endemic 
Some mechanical cause must be responsible, and for that cause I have 
suggested age. 
‘ Several botanists, while admitting that my figures are not to be gain- 
said, are of opinion that my results can be equally well explained by 
reversing my hypothesis, and considering that youth, rather than age, is 
responsible for the occupation of large area. I shall try to show that this 
conception leads logically to an untenable position, and shall also give some 
crucial test cases, which speak in favour of age. 
‘ Age is an obvious reason for occupying a large area, but youth is not, 
and we shall require stronger evidence to prove the latter. Obviously it 
must not be pushed to extremes, and a supplementary hypothesis will be 
needed to account for the fact that the very latest arrivals are not the 
commonest species. 
‘My hypothesis is based upon age within the country . What the 
species may have done in the way of spreading in other countries, or where 
it was evolved, and when, is immaterial ; when it arrives in the country with 
which we are dealing, it commences to spread, if suitable to the climate and 
soil, and spreads over an area determined, so long as conditions remain 
constant, and except in so far as barriers of mountains, broad rivers, sudden 
changes of climate from one region to another close by, and the like, inter- 
fere, by the length of time during which it has been in the country . 1 
‘ If, however, one try to reverse the hypothesis, one has at once to make 
choice of two cases. Either the supposition must be that area occupied 
depends upon youth zvithin the country , or upon absolute youth. The exact 
reversal of my hypothesis of course gives the first case. 
‘ On either view a great difficulty arises from the fact that the wides 
and endemics both show a graduated order of rarity, the former from many 
of large area to few of small, and the latter in the reverse direction. This 
fact, which in the Ceylon flora depended upon estimates, shows with actual 
measurements for New Zealand. There is thus nothing for it but to admit 
that my hypothesis must be reversed in detail, and that the younger 
a species is, whether absolutely or within the country, the greater area will 
it occupy. The same thing follows necessarily from the fact that in Ceylon 
the species common also to Peninsular India are intermediate in rarity 
between the endemics and the wides. Whatever hypothesis be adopted, one 
must admit that these species are intermediate in youth as in rarity. 
‘ A great difficulty for the hypothesis of absolute youth is the fact that 
the range in Ceylon in no way corresponds with the range outside of it, as 
one might expect upon this supposition. Sanicula europea , for example, 
and many other species have a vast range outside of Ceylon, and on this 
hypothesis are therefore presumably very young, yet within Ceylon only 
1 It being of course understood that the law does not necessarily apply to individual cases, any 
more than Mendel’s Law, but to groups of allied forms. 
