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. From what little I know, I cannot help thinking that you underrate the effects of the physical conditions of life on these low organisms. But I fully admit that I can give no sort of answer to your objections; yet I must add that it would be marvellous if any man ever could, assuming for the moment that my theory is true. You beg the question, I think, in saying that Protococcus would be doomed to eternal similarity. Nor can you know that the first germ resembled a Protococcus or any other now living form.
Page 12 of your letter: There is nothing in my theory necessitating in each case progression of organisation, though Natural Selection tends in this line, and has generally thus acted. An animal, if it become fitted by selection to live the life, for instance, of a parasite, will generally become degraded. I have much regretted that I did not make this part of the subject clearer. I left out this and many other subjects, which I now see ought to have been introduced. I have inserted a discussion on this subject in the foreign editions. (110/7. In the third Edition a discussion on this point is added in Chapter IV.) In no case will any organic being tend to retrograde, unless such retrogradation be an advantage to its varying offspring; and it is difficult to see how going back to the structure of the unknown supposed original protozoon could ever be an advantage.
Page 13 of your letter: I have been more glad to read your discussion on "dominant" forms than any part of your letter. (110/8. Harvey writes: "Viewing organic nature in its widest aspect, I think it is unquestionable that the truly dominant races are not those of high, but those of low organisation"; and goes on to quote the potato disease, etc. In the third edition of the "Origin," page 56, a discussion is introduced defining the author's use of the term "dominant.") I can now see that I have not been cautious enough in confining my definition and meaning. I cannot say that you have altered my views. If Botrytis [Phytophthora] had exterminated the wild potato, a low form would have conquered a high; but I cannot remember that I have ever said (I am sure I never thought) that a low form would never conquer a high. I have expressly alluded to parasites half exterminating game-animals, and to the struggle for life being sometimes between forms as different as possible: for instance, between grasshoppers and herbivorous quadrupeds. Under the many conditions of life which this world affords, any group which is numerous in individuals and species and is widely distributed, may properly be called dominant. I never dreamed of considering that any one group, under all conditions and throughout the world, would be predominant. How could vertebrata be predominant under the conditions of life in which parasitic worms live? What good would their perfected senses and their intellect serve under such conditions? When I have spoken of dominant forms, it has been in relation to the multiplication of new specific forms, and the dominance of any one species has been relative generally to other members of the same group, or at least to beings exposed to similar conditions and coming into competition.