Thomas Henry Huxley was Charles Darwin's ardent supporter, giving himself the title of "Darwin's Bulldog". Huxley lamented that Darwin's findings were so obvious that he should have thought of it, himself, and volunteered to take on all comers, in verbal battles that made headlines, as the champion of less confrontational Darwin.
While he defended Darwin with everything he could, Huxley did express some reservations about a single point of Darwin's theories. Huxley didn't think enough time had passed for natural selection, alone, to account for such diversity of species. He also pointed to unnatural selection never producing anything except different breads, rather than different species. Huxley felt that there must have been periodic jumps.
Modern scientific studies in DNA have shown that Huxley was right. There have been jumps, identified as mutations, in the evolution of our DNA.
Thomas Huxley was a scientist, above all else. He saw the scientific method in picking apples at the market.  The agnosticism he defined was a belief in that scientific method, and it amounted to a form of demarcation. No objective testable evidence = a subjective unfalsifiable claim. Results: unscientific and inconclusive. No belief as to the truth, or falsehood, of the claim.
"I say, strive earnestly to learn something, not only of the results, but of the methods of science, and then apply those methods to all statements which offer themselves for your belief. If they will not stand that test, they are nought, let them come with what authority they may." 
"Agnosticism is of the essence of science, whether ancient or modern. It simply means that a man shall not say he knows or believes that which he has no scientific grounds for professing to know or believe." 
"Agnosticism, in fact, is not a creed, but a method, the essence of which lies in the rigorous application of a single principle. That principle is of great antiquity; it is as old as Socrates; as old as the writer who said, “Try all things, hold fast by that which is good” it is the foundation of the Reformation, which simply illustrated the axiom that every man should be able to give a reason for the faith that is in him; it is the great principle of Descartes; it is the fundamental axiom of modern science." 
"That which Agnostics deny and repudiate, as immoral, is the contrary doctrine, that there are propositions which men ought to believe, without logically satisfactory evidence; and that reprobation ought to attach to the profession of disbelief in such inadequately supported propositions." 
His new label, that was neither theism, the belief that god(s) exist, nor atheism, the belief that god(s) don't exist, caught on in a big way. A number of writers, at the turn of the 20th century, called it the "Age of Agnosticism".
"The intellectual revolution caused by scientific findings had made, in relation to religion, the latter half of the nineteenth century the Age of Agnosticism." 
"Seth went on to observe how noteworthy and surprising it was that an age which saw such remarkable achievements in science 'should be also the Age of Agnosticism, the epoch of the creed Ignoramus et ignorabimus'." 
"The present philosophical situation has become simply intolerable"; "so far ... as the social and moral interests of mankind are concerned," the nineteenth century had become "the AGE OF AGNOSTICISM." 
"If one were asked to name the two most characteristic intellectual attitudes of the latter half of the nineteenth century, one would probably be safe in answering - Evolutionism and Agnosticism." 
Einstein would self identify as an agnostic. Karl Popper, also a self identified agnostic, would later cement falsification and demarcation into the philosophy of science. Carl Sagan would also self identify as an agnostic. Modern, well known, agnostics include Neil deGrasse Tyson and Bill Nye.
The main aspect of Huxley's philosophy, suspending judgement without sufficient evidence, goes back beyond his favourite philosopher, David Hume, whom he called the "Prince of Agnostics", and has roots in Pyrrhonian scepticism, rather than Academic scepticism, which some people attach it to. 
↑ Fordham University's, Modern History Sourcebook:
Thomas Henry Huxley: The Method of Scientific Investigation, 1863 
↑ Science and Religion, Museum of Geology, The Builder vol. 17 (January 1859) 
↑ Agnosticism: A Symposium, The Agnostic Annual (1884)