Page:Popular Science Monthly Volume 65.djvu/222

218 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 comparable 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 combination 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 elucidate evolutionary problems experimentally. The existence of such elementary species is no novelty which de Vries has been the first to make us acquainted with. Linnæus 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