Chapter CLXX: Section 41: The supposition, that the general characters of the

rabbit's ovum were stamped upon it as an heritage from a period when the ancestors of the mammals were egg-laying reptiles, is strengthened by the fact that the two lowest and most reptile-like of all the mammalia, the duck-billed platypus and the echidna, have been shown to depart from the distinctive mammalian character, and to lay eggs. And, in further confirmation of this supposition, we find, in tracing the mammals and reptiles back through the geological record, that in the Permian and Triassic rocks there occur central forms which combine, in a most remarkable way, reptilian and mammalian characteristics.