June 2, 1916] 



SCIENCE 



787 



tions prevailing in the island, and at least not 

 better adapted than the species from which 

 they sprung. Or, in other words, that they 

 did not originate in advantageous response to 

 those local conditions. A large amount of facts 

 and considerations has been brought forward 

 by the author in order to justify this conclu- 

 sion. 



These conclusions provide us with a strong 

 argument against the hypothesis of a slow and 

 gradual evolution by small and almost invisible 

 steps, and for the theory of their production 

 by mutations. In the rare cases of rapid dis- 

 persal of new species a better adaptation may 

 of course be assumed as one of the chief factors, 

 but on the average the dispersal is very slow 

 in the beginning, giving no argument in favor 

 of this view. 



Furthermore these considerations lead to the 

 view that wide distribution and commonness 

 are chiefly dependent on age, and only rarely 

 on adaptation. In every family the genera 

 with the widest distribution may be considered 

 as the oldest, those with a smaller domain as 

 younger, and the local endemics as the young- 

 est of all. These principles will be used in sub- 

 sequent studies to draw pedigrees of families. 

 But the studies made by the author up to this 

 time go to show that nearly all families have 

 the same general type of distribution, that 

 evolution of forms is on the average indiffer- 

 ent, and that most of the so-called adaptations 

 are of no special advantage to their possessors. 



Another argument relates to the possible 

 size of mutations. It is often assumed that 

 mutations must of necessity be small, consider- 

 ing that it seems probable that only one unit- 

 factor will be changed at a time. This con- 

 ception seems to the author to be an unneces- 

 sary handicap to the theory of mutation and 

 he proposes that it should be replaced by the 

 hypothesis that no specific change is too great 

 to appear in one mutation. The difference be- 

 tween endemic species of Ceylon and their 

 nearest allies is often very large, as may be 

 deduced from the fact that they are accepted 

 as well-marked Linnean species by such author- 

 ities as Trimen and Hooker. But in many 

 cases they are even larger. For instance. 



Coleus elongatus, which occurs only on the top 

 of Eitigala and here only in about a dozen of 

 individuals, differs so much from all other 

 Colei, that it may well be regarded as sub- 

 generically distinct. And for the lY endemic 

 genera, which have only one species each, it 

 seems at least very probable that the whole 

 genus has arisen at a single step. 



In concluding I might state that my own 

 studies on the production of new forms among 

 the CTlnotheras have of late led me to the con- 

 clusion that mutations are in many cases of a 

 far more complicated nature than has been 

 assumed until now. Many of them, as for in- 

 stance the production of 0. ruhrinervis, 0. 

 nanella and 0. gig as, involve the simultaneous 

 change of two or more characters, in some 

 cases of quite a large number of unit-factors. 

 Why these changes should so regularly go to- 

 gether, we do not, as yet, know, but the fact 

 goes to increase the analogy between the ex- 

 perimental mutations of these plants and the 

 mutations in the wild condition of the Ceylon 

 endemics. 



From the facts adduced by Willis, and re- 

 viewed in this article, it seems obvious that 

 the parallelism of natural and experimental 

 mutations is a very close one. 



Hugo de Vkies 



electrical discharge between concen- 

 tric cylindrical electrodes 



In operating vacuum tubes we invariably 

 use an induction coil or an electrostatic ma- 

 chine. The discharge in either case is neve^ 

 quite steady and hence these methods of opera 

 tion do not lend themselves well to a critical 

 study of the growth of the cathode dark 

 spaces. A steady, and of course continuous, 

 discharge may be had if the current is drawn 

 from a high potential storage battery. Ordi- 

 narily it takes more cells than are available; 

 however, by a right choice of conditions a 

 rather extended study may be made with di- 

 rect current potentials of less than 1,000 volts. 

 The following experiments with concentric 

 cylindrical electrodes were performed recently 

 by the writer in class demonstration. 



The discharge vessel consists of an ordinary 



