B.Dorsal aspect.] We have in all these animal types nervous systems differentiated on distinctly different patterns, fully formed organs of circulation, digestion, excretion, and generation, complexly constructed eyes and other sense organs; in fact, all the most elaborate and complete animal structures built up, and not only once, for in the fishes and mollusca we have (as described in the third chapter of this work) the coincidence of the independently developed organs of sense attaining a nearly similar complexity in two quite distinct forms.

If, then, so small an advance {142} has been made in fishes, molluscs, and arthropods since the Upper Silurian deposits, it will probably be within the mark to consider that the period before those deposits (during which all these organs would, on the Darwinian theory, have slowly built up their different perfections and complexities) occupied time at least a hundredfold greater.
Now it will be a moderate computation to allow 25,000,000 years for the deposition of the strata down to and including the Upper Silurian.

If, then, the evolutionary work done during this deposition, only represents a hundredth part of the sum total, we shall require 2,500,000,000 (two thousand five hundred million) years for the complete development of the whole animal kingdom to its present state.

Even one quarter of this, however, would far exceed the time which physics and astronomy seem able to allow for the completion of the process.
Finally, a difficulty exists as to the reason of the absence of rich fossiliferous deposits in the oldest strata--if life was then as abundant and varied as, on the Darwinian theory, it must have been.

Mr.Darwin himself admits[141] "the case at present must remain inexplicable; and may be truly urged as a valid argument against the views" entertained in his book.
Thus, then, we find a wonderful (and on Darwinian principles an all but inexplicable) absence of minutely transitional forms.