To compare very small things with great, Lingula, etc., remaining nearly unaltered from the Silurian epoch to the present day, is like the dovecote pigeons still being identical with wild Rock-pigeons, whereas its "fancy" offspring have been immensely modified, and are still being modified, by means of artificial selection.
You put the difficulty of the first modification of the first protozoon admirably.

I assure you that immediately after the first edition was published this occurred to me, and I thought of inserting it in the second edition.

I did not, because we know not in the least what the first germ of life was, nor have we any fact at all to guide us in our speculations on the kind of change which its offspring underwent.

I dissent quite from what you say of the myriads of years it would take to people the world with such imagined protozoon.

In how very short a time Ehrenberg calculated that a single infusorium might make a cube of rock! A single cube on geometrical progression would make the solid globe in (I suppose) under a century.