It must, however, be remembered that, if the theory of genetic descent with accumulation of minute modifications be the true explanation of the production of new forms, it necessarily follows, that could a complete record be forthcoming of the ancestry of any actual species, there would be found in that pedigree no distinction of species or genera, for no sharply marked lines of limitation would be discoverable. It would be like the case of a man who had been photographed every hour of his life from birth to old age;--immense though the difference might be between the two extremes, the gradations of change would at all points pass as imperceptibly into one another as do the phases of the moon. This consideration is both fundamental and obvious, yet it would seem to be almost universally ignored. It appears to be thought that, in order to demonstrate the fact of evolution, all that is needed is to find a form here and there, in some sense intermediate between others,--like the reptilian birds already mentioned. This would imply that the course of Evolution must be like that of an army, making long marches from point to point, and traceable only by the remains of its camp-fires: whereas it should be as that of a glacier continuously creeping on, and leaving its tracks at one point as much as another. What are wanted, therefore, as evidence for Evolution, are not isolated specific forms uniting some characteristics of those which they are supposed to connect,--as Nelson's men-of-war form a stepping-stone between the vessels of the Norsemen and the ironclads of the present day,--but a series sufficient to show, or at least to indicate, that all changes have been gradual and insensible, without the introduction at any point of a new element. To pursue the illustration, such a new element would be gunpowder or steam in the evolution of the battle-ship, for by no mere development could bows or javelins produce a cannon, or sailing ships a steamboat. Therefore, in proportion as the geological record approaches completeness, its testimony,--if it is to be in favour of Evolution--must tend more and more in this direction, and unless, in some instance at least, clear evidence be discoverable of the melting of one form into another, it cannot possibly be said that we have sufficient proof that such a process ever occurred. Mere graduated resemblance of isolated forms does not necessarily imply such transmutation, as we see for example in the methodical progression of shape, exhibited by various crystals, and even more remarkably in the affinities which we can recognize among what we know as elementary substances. There is another important point to be borne in mind. According to the teaching of Evolutionists such as Darwin or Haeckel,[245] every Species has originated from a single ancestor,--or, as they should rather say, from a single pair.