506 * 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 Vilmorin, 

 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- 

 bihty 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 

 fixity to existing species. In support he brings forward the 

 modus Of er audi 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 equahty 

 over the surface of the globe ; so are some of the larger orders. 

 If, then, the existing species have originated in variation, the 



