THE president's ADDRESS. 29 



fairly hope to solve, just as there are many with which the 

 experimentalist alone is capable of dealing. Both branches of 

 biological investigation are essential to satisfactory progress, and 

 it is in large measure the failure to recognise this fact that has 

 led to the one-sided and apparently contradictory views on the 

 subject of evolution that are so prevalent to-day. 



Take, for example, the strongly contrasted opinions of 

 Darwin and de Vries with regard to the origin of species and, 

 indeed, with regard to evolution in general. Darwin, as is well 

 known, laid the chief stress upon what he called slow, successive 

 variations, accumulating in definite directions under the influ- 

 ence of natural selection. De Vries, on the other hand, con- 

 siders that species originate suddenly, in apparently spontaneous 

 variations of an abrupt character, or, as he terms them, mutations. 



Now it is very easy to observe the occurrence of mutations, 

 or even to cause them to appear experimentally, as Tower has 

 shown in the case of the potato-beetle. On the other hand it 

 is impossible, owing to the extreme slowness with which the 

 necessary changes take place, to demonstrate the actual tran- 

 sition of one species into another by slow, successive, or, as we 

 now call them, fluctuating variations. Breeding experiments with 

 regard to mutations give positive results, with regard to fluctua- 

 ting variations they will probably give negative results, but that 

 does not prove that fluctuating variations have played no part in 

 evolution. We must seek other evidence on this point, and that 

 other evidence is supplied by the labours of the systematist. 



The most convincing instance of the part played by slow, 

 successive variation in the production of new species with which 

 I am acquainted is that of the flightless birds on oceanic islands. 

 All zoologists are agreed that these birds are the descendants of 

 ancestors which reached the islands in question by flying, and 

 which must, consequently, have had well-developed wings. 

 The loss of the power of flight can only be explained as due to the 

 disuse and consequent degeneration of the wings, flight being no 

 longer necessary as a means of escape from enemies. Now 

 degeneration, resulting from disuse, appears to be a gradual 

 process and, so far as I know, never takes place by mutation. 

 Yet this process, continued for many generations, has resulted 



