There is a small problem though.

Darwin never wrote a book called "My Life and Letters".

_The Life and Letters of Charles Darwin_, and _More Letters of
Charles Darwin_ (two volumes each) were published in 1887 and 1902
respectively.  (Charles Darwin died in 1882.) They were assembled
by his son Francis and A. C. Seward.  Since they are of such wide
interest, you can download all four volumes from Project Gutenberg.
I just did.  One advantage of having electronic copies is that it
is rather trivial to use search software to find quotes.

Having had no luck with finding the entire quote, I just searched
for a couple of word pairs.  Of the first half, I was unable to
find any trace.   No pair of significant adjacent words from the
first have seem to be found in any of the four volumes.  You are
free to try your own hand at it if you like.  You'll find the second
half (inaccurately) quoted in volume 2 of "Life and Letters of
Charles Darwin", in a letter to G. Bentham, May 22, 1863.

I'll quote the entirety of the letter, and put the quoted material
in all caps.

CHARLES DARWIN TO G. BENTHAM.
Down, May 22 [1863].

        My dear Bentham,

        I am much obliged for your kind and interesting letter.  I
        have no fear of anything that a man like you will say
        annoying me in the very least degree.  On the other hand,
        any approval from one whose judgment and knowledge I have
        for many years so sincerely respected, will gratify me much.
        The objection which you well put, of certain forms remaining
        unaltered through long time and space, is no doubt formidable
        in appearance, and to a certain extent in reality according
        to my judgment.  But does not the difficulty rest much on
        our silently assuming that we know more than we do?  I have
        literally found nothing so difficult as to try and always
        remember our ignorance.  I am never weary, when walking in
        any new adjoining district or country, of reflecting how
        absolutely ignorant we are why certain old plants are not
        there present, and other new ones are, and others in different
        proportions.  If we once fully feel this, then in judging
        the theory of Natural Selection, which implies that a form
        will remain unaltered unless some alteration be to its
        benefit, is it so very wonderful that some forms should
        change much slower and much less, and some few should have
        changed not at all under conditions which to us (who really
        know nothing what are the important conditions) seem very
        different.  Certainly a priori we might have anticipated
        that all the plants anciently introduced into Australia
        would have undergone some modification; but the fact that
        they have not been modified does not seem to me a difficulty
        of weight enough to shake a belief grounded on other
        arguments.  I have expressed myself miserably, but I am far
        from well to-day.

        I am very glad that you are going to allude to Pasteur; I
        was struck with infinite admiration at his work.  With
        cordial thanks, believe me, dear Bentham,

                        Yours very sincerely,
                                CH. DARWIN.

        P.S.--In fact, the belief in Natural Selection must at
        present be grounded entirely on general considerations.
        (1)  On its being a vera causa, from the struggle for
        existence; and the certain geological fact that species do
        somehow change.  (2)  From the analogy of change under
        domestication by man's selection.  (3)  And chiefly from
        this view connecting under an intelligible point of view a
        host of facts.  When we descend to details, we can prove
        that no one species has changed [i.e. WE CANNOT PROVE THAT  <= here
        A SINGLE SPECIES HAS CHANGED]; nor can we prove that the
        supposed changes are beneficial, which is the groundwork
        of the theory.  Nor can we explain why some species have
        changed and others have not.  The latter case seems to me
        hardly more difficult to understand precisely and in detail
        than the former case of supposed change.  Bronn may ask in
        vain, the old creationist school and the new school, why
        one mouse has longer ears than another mouse, and one plant
        more pointed leaves than another plant.

[ note: the phrasing around the quoted section is confusing, whether this
  is an error in transcription by the Project Gutenberg version or not, I
  cannot tell.  Anyone have a real copy lying around? ]

Darwin here admitted that he had not observed a single speciation event.
Indeed, he had no way of determining that the varieties and/or species
of an animal were in any way related via common descent.  He believed
it merely on the strength of the connection of "a host of facts" under
"an intelligible point".  

Of course now we actually have observed speciation, both in the wild and
in the laboratory, so modern scientists have no lack of direct evidence
that Darwin's insights were keen.

I found another interesting quotation while searching for "into another",
quoted below...

        I am actually weary of telling people that I do not pretend
        to adduce direct evidence of one species changing into
        another, but that I believe that this view in the main is
        correct, because so many phenomena can be thus grouped
        together and explained.  But it is generally of no use; I
        cannot make persons see this.  I generally throw in their
        teeth the universally admitted theory of the undulation of
        light,--neither the undulation nor the very existence of
        ether being proved, yet admitted because the view explains
        so much.

I find this interesting in two ways: because it freely admits that Darwin
believed (although had no direct evidence for) the transmutation of species
into another, and because of course it refers to the wave theory of light
and the ether, whose existence was actually _disproved_ in 1879 by the m
Michelson Morley experiments  (the letter above was written in 1861).