The out of context quote from the Darwin letter to Asa Gray is disingenuous since it implies that Darwin is "conceding" that evolution is "speculation". That is an utterly erroneous conclusion drawn by creationists because they cannot support their position using the scientific method so instead they rely upon distortion and dishonesty to achieve their nefarious anti-science agenda. There was a a great deal of correspondence between Darwin and Gray for many years and it focused around the concept of what today is called "intelligent design". Darwin was at a disadvantage because he did not have the science of DNA to support his position and if he had it would have resolved his dilemma. However he did not and thus engaged in the correspondence debate with Gray to try and resolve the problem. This correspondence was researched by Dr Sara Joan Miles and I recommend reading her papers because she lays out the arguments, both pro and con, in a fair and logical manner for each side. It was in her summation that she explains why Darwin would have written as he did to Asa Gray. http://www.asa3.org/ASA/PSCF/2001/PSCF9-01Miles.html The singling out of that one sentence from the mass of correspondence is like picking up a lone pebble on a beach and saying that it is "evidence" for an "intelligent designer". No, instead it is evidence of desperation on the part of creationists who cannot reconcile the fact that science negates the literal interpretation of the bible. Darwin was never able to accept the bizarre notion of an "intelligent designer" because logically that would mean that the "creator" had maliciously designed evil, pain and suffering into this world. Your question asking for the alternative to evolution is what pushes back and exposes that fallacy on the part of the creationists. They know that if they try and use the "intelligent designer" alternative they will then be faced with explaining away the evil, pain and suffering inherent in the "design". Hence their obfuscation and dissembling as a means to balance themselves between their faith and reality. For the creationists science denial is a palliative to avoid addressing the fact that biblical literalism is untenable and regressive in a modern world where knowledge is now ubiquitous. Ironically they are in the same situation that Darwin was in when he was writing those letters to Asa Gray. Theology was preaching something that could not be reconciled with the scientific reality of the world. Darwin, to his credit, went with the scientific knowledge while still respecting the right of good scientists of faith like Gray to believe as they wished. Even today there are such scientists working in the field and making evolutionary discoveries that prove Darwin was right. They just don't allow their "blind faith" to stand in the way of the scientific method.