Shortly after the Butler Act was ratified the ACLU released, through various Tennessee newspapers, a call for a local teacher, working within the public-funded school system, who would be prepared to go for trial for having violated the law.  The response to the advertisement was almost immediate, but it wasn't from a disaffected school teacher.  On the contrary, it was from a group of prominent Dayton citizens, led by the superintendent of the Sunday School at the local Methodist church.

It has often been claimed that William Jennings Bryan helped to draft the Butler Act, or at least persuaded Butler to putting the Act up for adoption by the Tennessee legislature as part of a wider ant-evolution crusade.  Not only is this untrue, however, it also misrepresents Bryyan's view on the teaching of evolution in schools.
For a more accurate picture of the situation we only need to look back to 1923, when Bryan did consult with the Florida legislature on the framing of a non-binding resolution, with no penalty attached, which spoke out against the teaching of evolution as a proven fact.

Contrary to the his fictional counterpart, Matthew Harrison Brady, Bryan was well-informed about science, at least as far as it related to evolution as it was understood in 1925.  And although he objected to the teaching that men were descended from some ape-like ancestor on the grounds that it undermined the more "uplifting" version of Man's origins, as described in Genesis, this was certainly not his whole case.
He also regarded the "science" behind evolutionism as being little more than guesswork - which at that time it certainly was (the description of the supposed ancestry of human beings, for example, was highly flawed) - and liable to encourage a brutalistic attitude in society that favoured the "survival of the fittest" attitude found in "social" Darwinism.  He also genuinely believed in the right of the people to determine what should be taught in the schools their taxes paid for.  As he told the West Virginia Legislature, also in 1923:

"The hand that writes the paycheck rukes the school."

Having said all that, Bryan had read Darwin's Origin of Species and The Descent of Man, and was able to quote from them - as he did during the trial.  He most certainly was not against the teaching of the sciences in general, nor even against the teaching of evolution in particular - so long as it was presented as a hypothesis.

After a certain amount of hesitation Scopes eventually agreed to the plan and Rappelyea sent a telegram to the ACLU to tell them that he'd found their man.  He also swore out a warrant against Scopes, who was actually arrested two days later, on May 7th, on the charge of having contravened the Butler Act.  He was immediately released on bail.
A preliminary hearing on May 10th bound Scopes over pending a Grand Jury hearing on May 25th.  The Grand Jury, who are well aware of the true purpose of the charge against Scopes, handed down an indictment and the trial date was set for July 10th.

John Godsey, in the meantime, came close to upsetting the whole applecart when, on June 13th he entered a motion with the court seeking to have the charge against Scopes quashed on the basis that the Butler Act was unconstitutional.  The Drug Store Conspirators, and their supporters, must have breathed a deep sigh of relief when the motion was turned down.

The announcement by Darrow, along with New York lawyers Dudley Malone and Bainbridge Colby, that they intended to offer their services to the defense team might have seemed like a godsend, to an outsider.  In reality, though, the ACLU didn't want Darrow or Malone anywhere near the defence team, either at the original trial or at the appeal.  But Darrow wasn't particularly concerned with what the ACLU wanted (as he explained in his autobiography, The Story of My Life, he was on a personal crusade against fundamentalists, and that was his sole object in offering his services).  Since the final choice was in the hands of the defendant, Scopes, the ACLU had to like it or back out of the whole affair.  They couldn't even refuse to pay Darrow and Malone's costs because both men had volunteered their services without charge.  (In Darrow's case it is interesting to note that despite his carefully nurtured reputation as the champion of the underdog, this was the one and only time in his professional career that he went so far as to waive his fee.)  Since they couldn't afford the kind of publicity which would have attended a climb down, the ACLU were forced to continue their support of Scopes, despite the presence of Darrow and Malone.

The reason behind the ACLU's thinking was that to suit their ends the trial must appear to be free of any religious, or anti-religious, bias.  On this basis the three leading ACLU figures connected with the case - Associate Director Forrest Bailey, Harvard Law Professor Felix Frankfurter and ACLU General Counsel Walter Nelles - would have preferred to use a true blue WASP (white, Anglo-Saxon Protestant), someone with an unsullied reputation in terms of both the law and religion.  Someone like John W. Davies or Charles Evans Hughes (both former presidential candidates); or Senator Thomas J. Walsh, who had led the investigation into the Teapot Dome Scandal; or Frank P. Walsh, co-Chairman of the War Labour Board during the First World War.

On May 28th, however, John Neal muddied the waters still further by simultaneously announcing that:

• The defence team would seek to secure the services of John W. Davies (one of the ACLU's preferred candidates), and
• John Scopes was entitled to choose whoever he liked to serve as his attorneys, and that both he (Neal) and the defendent (Scopes) "greatly desired" to avail themselves of the services of Darrow and Malone.

Not surprisingly, perhaps, Davis ignored the request by Neal, preferring instead to make the rather pointed announcement, on May 30th, that he wouldn't go to Dayton even if  he should be invited to do so.
The barely discreet manouevring continued through to June 6th, when Scopes and Neal travelled to New York to confer with ACLU officials.  Officially the main topic of discussion was trial tactics.  Unofficially Scopes was pressured to reject Darrow and Malone and accept a less controversial defence team.  Scopes, no doubt impressed by Darrow's defence of the child murderers Loeb and Leopold the previous year, remained deaf to all appeals, and on June 8th Forrest Bailey made a statement to the Press specifying that the defence team would consist of Darrow (chief counsel), assisted by Bainbridge Colby, John Neal, Dudley Malone, and another ACLU stalwart, Samuel Rosensohn.

Even then the matter wasn't finally settled.  First, the ACLU tried to tell Malone that he should remain in New York doing research rather than appearing in Dayton.  Malone pointed out that he had clerks to do whatever research was necessary and that he had every intention of joining the defence team in Dayton when the time came.
Colby and Rosensohn were a different matter, however.  Both men retired from the defence and were replaced by Arthur Garfield Hays, another ACLU lawyer and a keen advocate of the ACLU's brand of free speech (of which more will be said later).

The final lineup for the Defense included Clarence Darrow, Dudley Field Malone, John Neal, ACLU representative and defense team manager Arthur Garfield Hays, and local attorney Frank B. McElwee

Key Members of the Defense Team
Left to right: Darrow, Malone, Neal and Scopes

The prosecution team had grown in a similarly haphazard but considerably less acrimonious fashion.

On May 13th, W.B. Riley and other leading members of the WFCA (World's Christian Fundamentals Association), who were meeting in Memphis at the time, sent Bryan a telegram asking him to represent them at the trial.  On the 14th, Sue Hicks, prompted by an enquiry from the Memphis Press regarding the WCFA approach to Bryan, eagerly sent off a letter of his own, in which he assured Bryan that: "We will consider it a great honor to have you with us in the prosecution."
Hicks needn't have worried.  By the time his letter arrived, Bryan had already made a public announcement saying that if the official prosecutor would accept him, he would indeed represent the interests of the WFCA at Scopes' trial.  This, in turn, is generally believed to have been what triggered the entrance of Clarence Darrow into the proceedings.  (Some commentators have suggested that it was actually journalist H.L. Mencken who first heard of Bryan's announcement and subsequently persuaded Darrow to get involved.  However another source claims that Mencken actually made the suggestion at a conference staged in Richmond, Virginia by the American Psychological Association, where Darrow was a speaker - before Bryan became involved - and Darrow dismissed the suggestion on the grounds that he had only just announced his retirement.)
It should be noted, here, that despite a welter of claims to the contrary, Bryan was accepted as one of several assistant prosecutors, and not as "the" prosecuting counsel.