Il6 PHILOSOPHICAL TRANSACTIONS. [ANNO J 797. 



gratuitous, should be received with caution and distrust. Before any deduction can 

 be made from analogy concerning the means by which any important end is to be 

 effected, we cannot examine the instruments performing such actions with an at- 

 tention too nice or too minute. If we find nature employing different instruments, 

 in different animals, to produce the same ultimate effect, I think it but fair to con- 

 clude, that the means used are essentially different; but the closer the resemblance 

 in the instruments or organs, the nearer will the means approach. On this prin- 

 ciple no conclusions can be drawn respecting the human species, from observations 

 either on vegetables, or even on frogs, toads, and newts. Birds, as being impreg- 

 nated by semen conveyed into the body, resemble human impregnation more than 

 the former: but they differ so obviously in the mode of perfecting the foetus from 

 the ovum, that I hardly dare rest any thing on their general analogy. There is 

 however a curious fact respecting them not altogether inapposite to this question, 

 which is, the permanent effect of one coitus. I have read in the Abbe Spallanzani's 

 dissertation, and elsewhere, that all the eggs which a hen will lay in 20 days will 

 be impregnated at one coitus: and Mr. Cline tells me, that in Norfolk this matter 

 is reduced to a certainty with respect to turkeys; and that even to a greater 

 extent. There is certainly some difficulty in reconciling these facts to impregnation 

 by contact of semen; but from the very obvious difference between oviparous and 

 viviparous animals, I shall not press this argument further. Indeed it should al- 

 ways be impressed on the recollection of those who are labouring in the pursuit of 

 truth, that arguments drawn from analogies, unless from those of the nearest re- 

 lation, are better adapted to the purpose of illustration than of proof: and though 

 they frequently find advocates in confident closet philosophers, they are received 

 with deserved distrust by the more cautious practical physiologists. 



Those who cannot admit the passage of semen by the tubes, do not neglect to 

 take the advantage of some difficulties which their opponents have overlooked. 

 They say, implicit confidence is not due to the observations of Morgagni and 

 Ruysch, and that what appeared to them to be semen in the uterus and tubes, was 

 nothing more than the mucus of the parts. They further invalidate the force of 

 this argument by contrasting these solitary observations, with a numerous train of 

 counterfacts ; for in all the experiments made by Harvey, De Graaff, Haller, and 

 others, it does not appear that semen was found beyond the vagina, except in one 

 of Haller's experiments in a sheep, in which he saw semen in the uterus 45 minutes 

 after coition. But this fact stands almost alone; and when placed in opposition to 

 the many experiments attended with a contrary result, will weigh but little in the 

 balance of impartial decision. Yet he rested much on this one fact, and adduced 

 it in support of his opinion, that whenever impregnation happened, the semen 

 passed into the uterus, and was retained; but when it returned from the vagina, 

 then the animal remained uni impregnated. In this latter case, he supposes the 

 semen had never passed beyond the vagina; for if it had, he says it would have: 

 been retained. This argument he thinks is unanswerable. 



