concerning Animal Impregnation. 169 
the satisfaction of stern philosophers, that the pollen pervades 
the pistillum , and stimulates the contents of the pericarpium by 
contact, to the evolution of the germen. Such would deny the 
contact of semen. The advocates for either opinion then may 
avail themselves of analogies suited to their own mode of think- 
ing. It may be said, however, and with some colour of truth, 
that the latter analogy, as being more remote than the former, 
and as being founded on a principle which some have suspected 
to be 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 attention too nice or too minute. If we find nature employ- 
ing different instruments, in different animals, to produce the 
same ultimate effect, I think it but fair to conclude, that the 
means used are essentially different ; but the closer the resem- 
blance in the instruments or organs, the nearer will the means 
approach. On this principle no conclusions can be drawn re- 
specting the human species, from observations either on vege- 
tables, or even on frogs, toads, and newts. Birds, as being im- 
pregnated by semen conveyed into the body, resemble human 
impregnation more than the former ; but they differ so obvi- 
ously in the mode of perfecting the foetus from the ovum, 
that I scarcely dare to 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 Abb6 Spallanzani's disserta- 
tion, and elsewhere, that all the eggs which a hen will lay in 
twenty days will be impregnated at one coitus : and Mr. Cline 
tells me, that in Norfolk this matter is reduced to a certainty 
mdccxcvii. Z 
