218 POPULAR SCIENCE MONTHLY. 



pendent creation of every species at least furnished us with a distinct 

 definition, even though transcendental. But as the idea of a slow and 

 gradual evolution in nature has come to predominate in the course of 

 time, sharp boundary lines have been effaced, and species have become 

 both of an artificial and of a temporary nature. They have become a 

 compartment into which man temporarily brings together a larger or 

 smaller number of individuals, knowing that in times gone by the 

 contents were confluent with those of another such compartment and 

 that in the far future there will be other changes. 



And when in later years the ideas of Wallace, which we mentioned 

 above, found increasing sympathy, the lines of separation grew dimmer 

 yet, and the idea of species became exclusively an artificial limit com- 

 parable in musical terms to so many thin lines that mark the bars in 

 the continuous symphony of the evolution of life upon earth. 



Thanks to de Vries's experiments, which have enabled him to 

 formulate his mutation theory, he has now provided us with the means 

 to define species more strictly. The species is limited by space and 

 time. By time, for it begins whenever, by a process of mutation, its 

 peculiar combination of specific characters springs into existence, even 

 though this mutation, unobserved by the untrained eye, can as yet only 

 be detected by the specialist. Whenever mutation appears, the com- 

 bination of specific characters is modified in the mutants, and at the 

 same time a new species has appeared side by side with the mother 

 species, which itself remains stable. 



The distribution of a species in space can be very varied; some are 

 known only from a very limited area, others may be cosmopolitan. 

 The species thus limited in time and space is what de Vries calls an 

 elementary species. It is with these elementary species that the next 

 generation of naturalists will have to grapple when they wish to eluci- 

 date evolutionary problems experimentally. The existence of such ele- 

 mentary species is no novelty which de Vries has been the first to make 

 us acquainted with. Linnaeus knew these elementary species perfectly 

 well, but he called them varieties and forbade his pupils to waste their 

 time on them. 'Varietates levissimas non curat botanicus.' From 

 his point of view this was perfectly justified. He came forward to 

 restore order in the chaos of classification, and as such he strove to 

 combine the material then available into not too small bundles. His 

 species were what the Germans have called by an expressive name 

 ' Sammelarten, ' receptacles, into which the so-called 'varietates minores' 

 were thrown together. According to his idea, the species had been 

 created in the beginning as an entity, the 'varietates' had gradually 

 arisen from it, even though he could not prove this experimentally. 

 With regard to them, Linnaeus was an evolutionist, just as, among his 

 predecessors, the idea had long predominated that the genera had been 



