light the tenuity of the distinction between species and varieties. The fact of embryology, the occurrence of rudimentary organs, and the fundamental unity of structure which obtains in vast groups, such as the vertebrata and arthropoda, all tended to suggest the existence of a genetic connection between species, so that Lamarck was finally led to renounce the doctrine of the fixity of species, and to define a species as "a collection of individuals which resemble each other and produce their like by generation, so long as the surrounding conditions do not alter to such an extent as to cause their habits, characters, and forms, to vary."
According to this definition the distinction between species and variety once more becomes conventional. A variety is, in fact, a nascent species; and the notion of the creation of species vanishes, inasmuch as every species is the result of the modification of a predecessor. Lamarck's views of the nature of geological changes were in harmony with his biological speculations, and wholesale catastrophic revolutions were as completely excluded from the one as from the other.
It is impossible to read the "Discours sur les Révolutions" of Cuvier, and the "Principes" of Lamarck, without being struck with the superiority of the former in sobriety of thought, precision of statement, and coolness of judgment. And it is no less impossible to consider the present state of biological science without being impressed by the circumstance that it is the conception of Lamarck which has triumphed, and that of Cuvier which has been utterly vanquished.
Catastrophic geology has vanished out of sight, and is everywhere replaced by the conception of slow and gradual change. With it has disappeared the once prevalent notion that the whole living population of the earth has been swept away and replaced in successive epochs. On the contrary, it is now well established that the changes which have taken place in that population have been effected by the slow and gradual substitution of species for species.
Moreover, it is well established that in some cases the succession of forms in time is the same as that which should have occurred if the hypothesis of evolution is correct.
The rapid advance of comparative anatomy has diminished or removed the wide intervals which formerly appeared to separate the different divisions of the animal and vegetable kingdoms from one another. Even the hiatus between the vertebrata and the invertebrata is bridged over by recent discovery. The establishment of the cell-theory, however much the views originally propounded by Schwann have been modified, leaves no doubt that there is a fundamental similarity in minute structure, not only between all animals, but between them and plants, while the discoveries of embryologists have proved that even the most complex forms of living beings do, in
