[143] We prefer this unpretending definition of the theory of descent to every other. As soon as one introduces into the definition the concept of the “transmutability of species,” the term “species” would require a special definition, and that would lead to difficulties which it is unnecessary to deal with for our main purposes. It has been remarked by Krašan, (*Ausichten und Gespräche über die individuelle und specifische Gestaltung in der Natur*) and by several other writers, that the problem of mutability or immutability of course relates to the individuals in the first place. I should like to add to this remark that the possibility must be admitted of the individuals being transmutable, whilst the “species” are not transmutable at the same time, the line of the “species” being a fixed order, through which the “individuals” have to pass in the course of their generations. What is meant here will become clearer, when we study the different possible aspects of “phylogeny.” There are two different groups of facts which have suggested the idea of transformism: none of these facts can be said to be conclusive, but there certainly is a great amount of probability in the whole if taken together. The first group of evidences which lead to the hypothesis of the real relationship of organisms consists of facts relating to the geographical distribution of animals and plants and to palæontology. As to geography, it seems to me that the results of the floral and faunal study of groups of islands are to be mentioned in the first place. If, indeed, on each of the different islands, *A* *B* *C* and *D*, forming a group, the species of a certain genus of animals or plants are different in a certain respect, and show differences also compared with the species living on the neighbouring continent, of which there is geological evidence that the islands once formed a part, whilst there is no change in the species on the continent itself for very wide areas, then, no doubt, the hypothesis that all these differing species once had a common origin, the hypothesis that there is a certain community among them all, will serve to elucidate in some way what would seem to be very abstruse without it. And the same is true of the facts of palaeontology. In the geological strata, forming a continuous series, you find a set of animals, always typical and specific for every single stratigraphical horizon, but forming a series just as do those horizons. Would not the whole aspect of these facts lose very much of its peculiarity if you were to introduce the hypothesis that the animals changed with the strata? The continuity of life, at least, would be guaranteed by such an assumption.