"Analogy would lead me to the belief that all animals and plants have
descended from one prototype."

Here are a few specimens, his illustrations of the theory:-

"There seems to me no great difficulty in believing that natural
selection has actually converted a swim-bladder into a lung or organ
used exclusively for respiration." "A swim-bladder has apparently
been converted into an air-breathing lung." "We must be cautious in
concluding that a bat could not have been formed by natural selection
from an animal which at first could only glide through the air." "I
can see no insuperable difficulty in further believing it possible
that the membrane-connected fingers and forearm of the galeopithecus
might be greatly lengthened by natural selection, and this, as far as
the organs of flight are concerned, would convert it into a bat."
"The framework of bones being the same in the hand of a man, wing of
a bat, fin of a porpoise, and leg of a horse, the same number of
vertebrae forming the neck of the giraffe and of the elephant, and
innumerable other such facts, at once explain themselves on the
theory of descent with slow and slight successive modifications."

I do not mean to go through your correspondent's letter, otherwise "I
could hardly reprehend in sufficiently strong terms" (and all that
sort of thing) the perversion of what I said about Giordano Bruno.
But "ex uno disce omnes"--I am, etc.,

"THE SAVOYARD."