genera as created, and species to have originated from them. When the number of known species increased and soon assumed undreamed of proportions, it seemed but natural not to accept for each species a separate creation. But the others denied the possibility of a transition from an old form to a new one by natural means. Each actually existing form, constant from seed, must, according to their idea, have been created as such. They denied the right to collect groups of forms under one specific name, as did Linnaeus and, after him, his disciples, especially when, owing to constant research, an exceedingly large number of forms became known. Instead, they recognized each form as a unity—unities which could be collected under a generic name only.
But Linnaeus, guided more by the talents of a lawyer than by those of an investigator, had once for all connected his conception of a species with the use of the binomials introduced by him. Whatever bears two names is a species. This is the law which all must obey. Genera bear simple names, subdivisions of species tri-or quadrinomials. Whoever wishes to have a form recognized as a species, must give it two names. Unless this be done he will not attain his object. But the number of simple forms, constant from seed, increased year by year and, even for Europe alone, threatened to become ten times greater than it had been.
Since the validity of the theory of descent has been generally recognized, these questions have lost much of their importance.
The work of Darwin embraces two main theses which as a rule are not sufficiently distinguished and which even by him were frequently collated. The one was to ascertain the common descent of plants and animals, the other, to find how one species could have originated from another. These two points are mutually independent, and were especially so at the time of publication of Darwin's 'Origin of Species.'
The doctrine of the common descent of all organisms holds that genera, families and even the larger divisions of the plant-and animal-kingdoms originated in a manner identical with the one which, before the days of Linnæus, was largely accepted for the splitting of genera into species and afterwards for the formation of subspecies from species. The common origin of groups of smaller types was recognized; but how large these groups were no one knew exactly. Darwin extended their limits so as to enclose all living organisms, practically collecting them into a single genus.
For this purpose it was not even necessary to know how the simple forms themselves originated. What was conceded for these by every one, had only to be applied to the larger groups. Yet Darwin attached considerable weight to this question and threw much light upon it.
