The number of the entirely authenticated cases now known of the birth of fully developed children within from two to five months of each other, can leave no doubt as to the possibility of such an occurrence. The only question which remains is in regard to the periods of conception. Are the two children in such cases twins, conceived at the same time, but the growth of the last-born so retarded that it did not arrive at maturity until a number of months after its fellow? or, Has a second conception taken place at an interval of several months after the first? If this latter view be true, then, in the instance of Marie Anne Bigaud, above related, the second child must have been conceived after the first had quickened. Then, also, two children of different ages, the offspring of different fathers, may exist in the womb at the same time. The weight of scientific observation and authority has now established the fact that, in very rare instances, a second conception may take place during pregnancy. It must not be understood as necessarily following from this statement, that when two children are born at the same time,--one fully developed, and the other small and apparently prematurely born,--the two were conceived at different times. The smaller may have been blighted and its growth hindered by the same causes which bring about such effects in cases of single births of incompletely developed children. A similar supposition may account for the birth of a second child within a month or two after the first, for the first may have been prematurely born, and the second carried to full term. But no such supposition can explain the cases referred to, and others which might be mentioned, in which the interval has been five or six months, each child presenting every indication of perfect maturity. The only explanation possible in such instances, which, as has been said, are well authenticated, although few in number, is, that a second pregnancy has occurred during the first.