180 
LETTERS TO A STUDENT. 
ovaria. One of their vesicles is detached and conveyed to the 
uterus, where it undergoes several transformations. That the 
process originates in the presence of the semen is proved beyond 
doubt. There is no dispute about this. But, then, the semen 
must be conveyed to the ova upon which it is to act, and a 
passage is found by the Fallopian tubes ; but as there is some 
difficulty in shewing how these tubes are able to seize the fluid, 
other hypotheses have been invented. By some it is supposed 
that the semen is absorbed and carried to the ovaria by means 
of the blood, while others contend that “the subtile prolific aura 
of the semen virile,” not the semen itself, ascends and passes 
along the duct by which the ova is brought to the uterus. 
The process of impregnation is quite as lucid before as after 
these explanations. There is not, surely, any absurdity in sup- 
posing that the presence of the semen in the uterus is quite a 
sufficient cause of a change in the ovaria. This is all that we 
truly know, and, perhaps, there is nothing more to be known. 
An irritant applied to the glottis is instantly followed by a con- 
vulsive action of the abdominal and thoracic muscles ; but no one 
supposes that the irritant touches those parts. It is possible, 
indeed, that the semen may produce a particular state of the 
uterus, or of other parts, by which germination is excited in the 
ova ; but this is not what the theorists have been anxious to 
learn : the object of their investigations and reasoning has been 
to bring the semen and the ova into contact. 
It would be an easy task to prolong this subject. Pathology, 
not less than physiology, abounds with numerous illustrations. 
I need only allude to the vain efforts to explain the operation of 
such a substance as a poisonous dose of prussic acid. It is not 
more wonderful that death takes place when prussic acid is 
applied to the tongue or to the throat, than when applied at once 
to the brain. Possibly it may, or must, reach that organ ; but 
there is not the slightest reason for supposing that it does. 
There are very many errors of this kind in etiology. 
In considering the effects produced by external agents, you 
are to discover, as far as possible, the changes in the order of 
their succession. In general, the first change is the cause of 
the second, the second of the third, and so on till the last. 
These subsequent effects are entirely dependent each upon that 
which it follows ; and they occur whether the original cause 
does or does not exist. You must also remember, that substances, 
both animate and inanimate, operate upon each other at certain 
distances : in all cases there is some little or large space between 
them ; and within certain limits, the change or effect varies with 
the distance. When a blister is applied to the skin, it appears 
