Two relations of species to time.--No evidence of past existence of minutely intermediate forms when such might be expected _a priori_.--Bats, Pterodactyles, Dinosauria, and Birds.--Ichthyosauria, Chelonia, and Anoura.--Horse ancestry.--Labyrinthodonts and Trilobites.--Two subdivisions of the second relation of species to time.--Sir William Thomson's views.---Probable period required for ultimate specific evolution from primitive ancestral forms.--Geometrical increase of time required for rapidly multiplying increase of structural differences.--Proboscis monkey.--Time required for deposition of strata necessary for Darwinian evolution.--High organization of Silurian forms of life.--Absence of fossils in oldest rocks.--Summary and conclusion.
Two considerations present themselves with regard to the necessary relation of species to time if the theory of "Natural Selection" is valid and sufficient.
The first is with regard to the evidences of the past existence of intermediate form, their duration and succession.
The second is with regard to the total amount of time required for the evolution of all organic forms from a few original ones, and the bearing of other sciences on this question of time.
As to the first consideration, evidence is as yet against the modification of species by "Natural Selection" alone, because not only are minutely transitional forms generally absent, but they are absent in cases where we might certainly _a priori_ have expected them to be present. [Page 129]
Now it has been said:[125] "If Mr. Darwin's theory be true, the number of varieties differing one from another a very little must have been indefinitely great, so great indeed as probably far to exceed the number of individuals which have existed of any one variety. If this be true, it would be more probable that no two specimens preserved as fossils should be of one variety than that we should find a great many specimens collected from a very few varieties, provided, of course, the chances of preservation are equal for all individuals." "It is really strange that vast numbers of perfectly similar specimens should be found, the chances against their perpetuation as fossils are so great; but it is also very strange that the specimens should be so exactly alike as they are, if, in fact, they came and vanished by a gradual change."