170 Dr. Haighton's experimental Inquiry 
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 farther. Indeed it should always be im- 
pressed on the recollection of those who are labouring in the 
pursuit of truth, that arguments drawn from analogies, unless 
from those of the nearest relation, are better adapted to the 
purpose of illustration than of proof : and though they fre- 
quently find advocates in confident closet philosophers, they 
are received with deserved distrust by the more cautious prac- 
tical 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 Morgagm 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 Graaf, Hal- 
ler, and others, it does not appear that semen was found be- 
yond the vagina, except in one of Baron Haller's experiments 
in a sheep, in which he saw semen in the uterus forty-five 
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, however, he rested much upon this 
one fact, and adduced it in support of his opinion, that when- 
ever impregnation happened, the semen passed into the uterus. 
