He acknowledged this elsewhere in The Origin of Species: "As to the question of why we can't find a rich reservoir of fossils dating back to the supposed pre-Cambrian rock system, I can't give a satisfactory answer.
Darwin himself acknowledged the validity of Agassiz's objection; he acknowledged this elsewhere in Origin of Species: "As for the question why we can't find a rich reservoir of fossils dating back to the initial epochs assumed before the Cambrian rock system, I cannot give a satisfactory answer ... that case must remain for the time being unexplainable, and may indeed seem a valid argument against the concepts we have advanced here."
Darwin's attempts to explain the absence of predicted ancestral fossils of Cambrian forms failed to meet Agasis's strong and intelligent objections; the dilemma in Darwin's theory, as explained by Agasis, was not the general lack of the fossil record, or the widespread absences of ancestral forms of life in the fossil record, but rather the dilemma, according to Agasis, of selective discontinuity in the fossil record.
Agassis asks: Why does the fossil record always break at the bonding nodes of the large branches of Darwin's tree of life, but it rarely breaks, in the style of modern paleontology, in the "finite branches" representing groups of known organisms? These finite branches are usually well represented in the fossil record, extending for many generations through millions of years, whereas the "inner branches" of the bonding nodes of Darwin's tree of life are always selectively missing.
If so, and regardless of his stated desire to wait for fossils in the future, Darwin did not have that answer.
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