50G * ORIGIN ' AND ' TASMANIAN FLORA ' 
This element of mutability pervades the whole vegetable 
kingdom ; no class nor order nor genus of more than a few 
species claims absolute exemption, whilst the grand total of 
unstable forms generally assumed to be species probably 
exceeds that of the stable. 
He adds a doctrine of ' centrifugal variation ' : 
The tendency of varieties, both in nature and under 
cultivation, when further varying, is rather to depart 
more and more widely from the original type, than to revert 
to it. 
In the New Zealand Flora he had quoted the current 
opinion of the tendency to reversion in cultivated stocks as 
supporting the theory of permanency in species. This, on 
further evidence, he now doubts. The reversion is one of 
habit, not of specific character. He agrees with Yilmorin, 
the famous horticulturist, that when once the constitution of 
a plant is so broken that variation is induced, it is easy to 
multiply the varieties in succeeding generations. 
On the other hand, if nature has provided for the possi- 
bility of indefinite variation, she regulates it as to extent and 
duration, by methods such as cross fertilisation, indicated by 
Darwin. Thus ' it is doubtful whether the natural operations 
of a plant tend most to induce or to oppose variation ' ; hence 
both views on species find support in nature, and the question 
cannot be decided by investigating variation alone. It is 
these checks on indefinite variation aided by the extinction of 
unprofitable varieties, that give a temporary appearance of 
tixity to existing species. In support he brings forward the 
modus o'perayidi of Natural Selection. 
The facts of distribution when analysed point in the same 
direction towards connected change. Species are replaced 
in distant areas by allied forms ; the same varieties do not 
appear to repeat themselves at different periods when the 
sum of conditions cannot have been identical. The three 
great classes of plants are distributed with tolerable equality 
over the surface of the globe ; so are some of the larger orders. 
If, then, the existing species have originated in variation, the 
