It would lead too far were I to enter here upon the various points on which their theories differ; let it be sufficient to note some parts of Scott's treatise, since this gives the sharpest and clearest contrast to the reigning view. In the long ancestral trees which have been brought to light by the study of prehistoric animals, one form leads gradually to another. When the strata are sufficiently known there remain no breaks in the pedigree. Breaks are met with only where the strata are wanting or where it has as yet been impossible to study them thoroughly. Each ancestral tree consists of an uninterrupted series of forms. Between two adjacent ones there exists no greater difference than between the two most closely related species of the present day. And in the successive strata they follow each other up in such a manner as corresponds to the gradual development of the ancestral tree.
But how did each form originate from the one immediately preceding it? Gradually or suddenly? Directly, paleontology can of course not teach us anything upon this subject. Did the species originate suddenly, then there can have been no intermediate forms, but even if they originated gradually the chances that such intermediate forms would have become fossilized, are exceedingly slight. For how small is the proportion of fossilized specimens to those which once must have existed! In any case, no such intermediate forms have been found, and it is for this reason that many paleontologists accept a sudden formation of new forms from the older ones. The transition is slight, as slight for instance as the well-known differences between the local races of slugs; but as these races are constant, so in paleontology are the closest related forms sharply separated from one another.
The contrast between the views of Scott and those of the majority of botanists and zoologists has, I believe, been sufficiently shown here. According to Scott species did not originate gradually, but by small jumps. By each jump a limit was passed, but after that the species remained constant until, perhaps many centuries later, a new shock produced a new form. Each species, each subspecies, or even each variety, is constant in all its characters; they remain the same from the beginning till the end, until, later on, either after having produced other species, or without having done so, they succumb in the struggle for life.
This theory restores the doctrine of the invariability of species to its old place. And this invariability is so general a matter of experience that it has always remained an exceedingly weak point of Darwin's theory of descent. The continual, slow, even inappreciable changes of species, which Darwin, but more especially Wallace and his disciples, accepted, and which are so lineally opposed to every-day expe-
