And also the hypothesis of Haeckel, that the cradle of mankind was a land between Africa and Asia, now sunk in the sea, and called Lemuria, can be neither proved nor denied. Such vague possibilities have indeed not the least scientific value. In considering these contradictory results of geological investigation, we dare not overlook three points: First, our knowledge of the crust of the globe is still {87} very fragmentary, and does not yet extend over the whole globe. Further, it lies in the nature of the case that the strata in mountain formations can only give a very incomplete picture of the whole variety of the real organic life which may have populated the earth and the sea. What a poor picture of the present plant and animal life would be offered, for instance, by the soil of our continents, the slime, sand, and pebbles of our coasts and of the bottoms of our lakes and seas, if we had to construct from them alone the fauna and flora of the present! A third, but purely hypothetical, consideration is rendered of importance particularly by Darwin and Haeckel; namely, that the forms of transition without doubt existed for a shorter period than those forms whose organization has established itself in fully developed species. Thus far we have directed our attention to inquiring how the organic individuals were originated--and have throughout observed a successive development; next, we have questioned geology--and here also have observed a progress in the appearance of the species, but have received at the same time contradictory answers to the question whether this progress presents itself as a gradual development of one species from another or as a sudden appearance. So the reasons for and against the evolution theory almost balance one another; and it is not improbable that the hypothesis of an origin of species through development will have to share its authority with the hypothesis of a descent of species through heterogenetic generation, as well as with the hypothesis of a primitive generation of lower organisms, still repeating itself at a later time. Thus for the origination of {88} groups lying nearer together, we have the evolution theory; for the other groups, and especially for the origination of types where no transitions to other types can be traced, the theory of the heterogenetic or primitive generation recommends itself; and both theories thus far are of a purely hypothetical nature.