The second (Can't do that) has no basis in the text.
I'll spell the second one out for you again.
we are forced in each case to look to species of the same group, that is to the collateral descendants from the same original parent-form, in order to see what gradations are possible, and for the chance of some gradations having been transmitted from the earlier stages of descent, in an unaltered or little altered condition. Amongst existing Vertebrata, we find but a small amount of gradation in the structure of the eye, and from fossil species we can learn nothing on this head.
In other words, he is forced to look to "species of the same group" to see the possibility of "some gradations having been transmitted from earlier stages of descent" and finds "but a small amount".