In the first place, they must be produced by natural action in pre-existing material, or by supernatural action.
For reasons to be given in the next chapter, the second hypothesis need not be considered.
If, then, new species are and have been evolved from pre-existing material, must that material have been organic or inorganic?
As before said, additional arguments have lately been brought forward to show that individual organisms do arise from a basis of in-organic material only. As, however, this at the most appears to be the case, if at all, only with the lowest and most minute organisms exclusively, the process cannot be observed, though it may perhaps be fairly inferred.
We may therefore, if for no other reason, dismiss the notion that highly organized animals and plants can be suddenly or gradually built up by any combination of physical forces and natural powers acting externally and internally upon and in merely inorganic material as a base.
But the question is, how have the highest kinds of animals and plants arisen? It seems impossible that they can have appeared otherwise than by the agency of antecedent organisms not greatly different from them.
A multitude of facts, ever increasing in number and importance, all point to such a mode of specific manifestation.
One very good example has been adduced by Professor Flower in the introductory lecture of his first Hunterian Course.[[234]] It is the reduction in size, to a greater or less degree, of the second and third digits of the foot in Australian marsupials, and this, in spite of the very different form and function of the foot in different groups of those animals.
A similarly significant evidence of relationship is afforded by processes of the zygomatic region of the skull in certain edentates existing and extinct.