The excerpt represents the core issue or deciding factor on which you must meditate, and is drawn from Expression of Emotion in Man and Animals by Charles Darwin:|
The action serves to make the animal appear larger and more
frightful to its enemies or rivals, and is generally accompanied
by various voluntary movements adapted for the same purpose,
and by the utterance of savage sounds. Mr. Bartlett,
who has had such wide experience with animals of all kinds,
does not doubt that this is the case; but it is a different
question whether the power of erection was primarily acquired
for this special purpose.
 I have given some details on this subject in my `Descent
of Man,' vol. i. pp. 352, 384.
I will first give a considerable body of facts showing
Expression of Emotion in Man and Animals