If this were so, it would necessarily follow that every new form, originating in some particular spot of earth, would very gradually spread thence to other regions, fighting its way along. As Mr. Darwin acknowledges,[246] "The development by this means (i.e. Natural Selection) of a group of forms, all of which are descended from some one progenitor, must have been an extremely slow process; and the progenitors must have lived long before their modified descendants." Of this gradual spread of new types there should, at least in some cases, be some palæontological evidence. It is likewise by no means easy to understand how species thus generated could stand solitary and isolated from kindred forms in the records of the earth. The pair of individuals which started a new persistent group,--its members all stamped with the same specific characters, while all around were in a state of flux and divergence,--differed from their immediate ancestors, as we have seen, only infinitesimally. They can have differed no more from many of their contemporaries, for all the lines of descent must ramify afresh in each generation, and so form a web rather than anything like a line. It is not very easy to understand how a pair here and there struck root and founded a species, while the thousands which jostled them round about failed to do so, for the others which survived longest must be supposed to have resembled them most nearly, and therefore to have participated in their advantages. At least, we should expect to find around them the débris of the multitude they vanquished in the struggle for existence. We are told, moreover, that, with hardly an exception, the organic forms found in a fossil state must be supposed to be the last of their special line of development, which terminated in them; so that neither can they be claimed as the direct ancestors of any other forms, fossil or living, nor can any others which are actually known be claimed as their progenitors. The genealogies supplied for almost all known species, extinct or existing, are admittedly conjectural, and as in the most famous instance of all, namely the supposed common ancestor of simians and men, the links are persistently "missing." Thus M. de Quatrefages, speaking of the human pedigree as set forth by Professor Haeckel, writes thus:[247] All species, existing or extinct, are said to have been preceded by _ancestral forms_ which have disappeared without leaving the slightest vestige behind them. The _amphioxus_ itself, which more than any other realizes the type of the group it represents, was preceded, according to Haeckel, by the _provertebrate_, which no man has ever seen, but of which, nevertheless, the Jena professor gives us a figure, and describes the anatomy.