We have thus presented the reader with a review of the different arguments which have been adopted by the partisans and opponents of this celebrated doctrine, and we have cited copious authorities with a view to enable the student to pursue the investigation to any extent which may be commensurate with his notions of its importance. We shall now conclude by observing that the following occurrences are essential to constitute a case of superfœtation.[442] 1. The pregnant woman must bear two children, each of a distinct age. 2. The delivery of these children must take place at different times, with a considerable interval between each. 3. The woman must be pregnant and a nurse at the same time. Q. 10. _What are the causes of Abortion?_ A gratuitous assumption on the part of some writers respecting the _viability_ of the fœtus, has led them to adopt a division into _abortion_ and _premature labour_, according as the exclusion from the uterus takes place before, or after, the sixth month of conception; and the distinction is now generally adopted. Natural abortion may be considered as arising either from accidental or constitutional causes; we shall hereafter consider the different modes by which the premature ejectment of a fœtus may be occasioned by art. The exciting causes of accidental abortion may, in general, be easily detected[443]; those giving rise to the constitutional kind are often more obscure, and without great attention, the woman will go on to miscarry until either sterility or some fatal disease be induced. In many cases there can be no peculiar pre-disposing cause; as, for instance, when it is produced by blows, rupture of the membranes, or accidental separation of the decidua; but where it occurs without any very perceptible exciting cause, it is allowable to infer that some pre-disposing state exists, and this frequently consists in an imperfect mode of uterine action, induced by age, former miscarriages, and other causes. It is well known that women can only bear children until a certain age, after which the uterus is no longer capable of performing the action of gestation, or of performing it properly; now it is observable, that this incapability or imperfection takes place sooner in those who are advanced in life before they marry, than in those who have married and begun to bear children earlier; thus we find, that a woman who marries at forty shall be very apt to miscarry; whereas, had she married at thirty, she might have borne children when older than forty, from which it may be inferred, that the organs of generation lose their power of acting properly sooner, if not employed, than in the connubial state.[444] We also find that one miscarriage renders the woman liable to the accident at the same period of utero-gestation in subsequent labours, and to such an extent is this susceptibility carried, that it is often difficult with every precaution, for a woman to go to the full time, after she has