224 OUTLINES OF EVOLUTIONARY BIOLOGY 



given time the species living on the earth would form naturally 

 limited groups, which we should be able to define more or less 

 sharply if we had sufficient knowledge of the then existing 

 fauna and flora to enable us to trace their boundaries. If, 

 however, we were to include extinct forms in our survey, it is 

 obvious that the number of existing species which we should 

 recognize would be inversely proportional to the extent of our 

 palaeontological information, for the gaps between the survivors 

 would gradually be filled up by the discovery of intermediate 

 fossil forms. 



Naturalists have long recognized the fact that there is another 

 way in which more or less sharply defined groups of individuals 

 may arise, and that is by the occasional and sudden appearance of 

 " sports," or " mutations " as they are now generally called, 1 which 

 " breed true," handing on their special peculiarities from genera- 

 tion to generation. Darwin was of opinion that such sports differ 

 only in degree from the ordinary variations to the accumulation of 

 which he attributed the chief importance in the evolution of species. 

 At the present day, as we have already had occasion to point out, 

 some observers, and especially Professor Hugo de Vries, 2 consider 

 them to be totally different in kind from fluctuating or small 

 successive variations, and regard them as affording the sole means 

 by which new species originate. 



According to the mutation theory any species may, from time 

 to time, throw off such sports, either singly or in groups, and they 

 are from the first distinguishable by well defined, though it may, 

 be minute, characters from the parent form. These mutations 

 are regarded by de Vries as constituting " elementary species," 

 which are already fully characterized and will not change again 

 unless by giving rise to fresh mutations. De Vries maintains 

 that many of the species of flowering plants as defined by 

 Linnaeus are really " aggregate species," each made up of a larger 

 or smaller number of such elementary species. The common 

 weed known as Draba verna, for example, regarded by Linnaeus 

 as constituting a single species, occurs under about 200 more or 

 less distinct forms. All of these so-called elementary species are 

 believed to come true from seed and to have arisen as sudden 

 mutations. 



1 Compare Chapter XI. 



a Vide " The Mutation Theory," by Hugo de Vries. English translation by 

 Farmer and Darbishire (Kegan Paul, Trench, Triibner & Co. Ld., London, 1910). 



