In embryology he has proved it to the satisfaction of the scientific world. When he applies it to sociology our Bellamys are looking backward to Sir Thomas More, and expect a sudden transformation to a Utopia, not unlike the change which the good old preachers used to tell us we would experience "in the twinkling of an eye."
Haeckel builds on Darwin and shows that as the Cirripedia which makes the bottom of the ocean, the coral "insect" which rears dangerous reefs and even mountain-ranges, and Rhizopods that make the chalk cliffs possible, did not change the earth's crust in the twinkling of an eye, so neither can the efforts of man instantly change the social condition. Souls do not make lightning changes. Karl Marx thought society would change in the twinkling of a ballot, but he was not a Monist, and therefore did not realize that humanity is a solidarity of souls, evolved from very lowly forms and still slowly ascending.
And the beauty of it is that the Marxians are helping the race to ascend, by supplying it an Ideal, even if they fail utterly to work their lightning change. In the end there is no defeat for any man or any thing. When men deserve the Ideal they will get it. So long as they prefer beer, tobacco, brawls and slums, these things will be supplied. When they get enough of these, something better will be evolved. The stupidity of George the Third was a necessary factor in the evolution of freedom for America. All is one; all is Good; and all is God.
The Marxians will eventually win, but by Fabian methods, and Socialism will come under another name. As opposed to Herbert Spencer, Haeckel does not admit the Unknowable, although, of course, he realizes the unknown. No man ever had a fuller faith, and if there is any such thing as a glorious deathbed it must come to men of this type who believe not only that all is well for themselves, but for every one else. How a deathbed could be "glorious" for a man who had perfect faith in his own salvation and an equally perfect faith in the damnation of most everybody else, is difficult to understand.
A true Monist would rather be in Hell asking for water than in Heaven denying it.
He loves humanity because he is Humanity, and he loves God because he is God. As a single drop of water mirrors the globe, so does a single man mirror the race. And the evolution, biological and sociological, of the man mirrors the evolution of the species.