Churchill, in his work on Midwifery, writing on this subject says: “In conclusion, I would say--(1) That the theory of superfœtation is _unnecessary_ to explain the birth of a mature fœtus and a blighted ovum, of a mature and immature fœtus born together or within a month of each other, or of fœtuses of different colours, as they may reasonably be supposed to be the product of one act of generation, or of two nearly contemporaneous. (2) That, in cases of double uterus, it is possible for a second conception to take place, and--judging from the subsequent birth of the second child in the only case on record--at a later period than the first. (3) That, in the remaining cases, where one mature child succeeded the birth of another after a considerable interval, we have no proof of a double uterus in any, and positive proof that in one case it was single; and that to the explanation of these cases no theory as yet advanced is adequate, that of superfœtation being opposed by physical difficulties which are unsurmountable in the present state of our knowledge.” The late Dr. Matthews Duncan has, however, shown that the mouth of the womb is not completely closed by conception, and the communication between the vagina and ovary is not destroyed for some months after impregnation, and that there is no impediment to the ascent of the spermatozoa. Galabin[16] records an instance of extra-uterine and uterine pregnancy occurring at the same time, the extra-uterine fœtus being advanced in development as compared with that in the uterus, and regards the condition as one of superfœtation. [16] _Manual of Midwifery_, 1886. The late Dr. Milne, while admitting this form of pregnancy as possible, though very rare, remarks: “This variety we should not think due so much to mechanical hindrances as to the absence of proper ovules. It would imply extraordinary vigour were perfect ovulation to be achieved for any length of time after impregnation.” IMPOTENCE AND STERILITY Evidence in relation to the above subjects may be required in actions for nullity of marriage, divorce, legitimacy, inheritance, pregnancy, and criminal assault. =Impotence.=--By impotence is meant the incapacity for sexual intercourse, and applies both to the male and female; but the term is more especially used in reference to the former.