forms in cyclical propagation arise through so-called “heterogeneous generation,” _i.e._ through sudden _per saltum_ transformation. It is excusable if philosophers to whom these facts are strange, or who have to take the trouble of working them up, should adduce alternation of generation as an instance of “heterogeneous generation,” but by naturalists this should be once and for ever abandoned. All other facts which have hitherto been referred to “heterogeneous generation” are still less explicable as such, inasmuch as they always relate to changes in single parts of an organism, such as the sudden change of fruit or flower in cultivated plants. The notion of _per saltum_ development, however, demands a total transformation--it comprises (as Von Hartmann quite correctly and logically admits) the idea of a _fixed specific type_ which can only be re-modelled _as a whole_, and cannot become modified piecemeal. It must further be added, that the observed variations which have arisen abruptly in single parts are not as a rule inherited:[308] fruit-trees are only propagated by grafting, _i.e._ by perpetuating the individual, and not by ordinary reproduction by seeds. Now, if we nowhere see sudden variations of large amount perpetuated by heredity, whilst we everywhere observe small variations which can all be inherited, must it not be concluded that _per saltum_ modification is not the means which Nature employs in transforming species, but that an accumulation of small variations takes place, these leading in time to large differences? Is it logical to reject the latter conclusion because our period of observation is too brief to enable us to directly follow long series of accumulations, whilst _per saltum_ variation is admitted, although unsupported by a single observation? As long as there remains any prospect of tracing large deviations to the continually observed phenomenon of small variations, I believe we have no right to resort to the purely hypothetical explanation afforded by _per saltum_ variations. But the hypothesis of “heterogeneous generation” is not only without a basis of facts--it can also be directly shown to be untenable. Since the operation of an internal power of transformation does not explain adaptation to the conditions of life, the claims of natural selection to explain these transformations must be admitted; but the co-operation of a phyletic vital force and natural selection is inconceivable if we imagine the modifications to occur _per saltum_.