"I occasionally play works by contemporary [20th century] composers, and for two reasons. First, to discourage the composer from writing any more, and secondly to remind myself how much I appreciate Beethoven."
-- Jascha Heifetz, violinist
Because of its inexhaustible lungs -- it's ability to sustain a note indefinitely -- and its sheer power and range which exceeds a symphony orchestra, composers are well aware that the organ is specially suited to the expression of contrapuntal music, and that a musical theme which lends itself well to such contrapuntal manipulation will have certain characteristics [See blog, Getting Started With Writing, Part XXXIV].
Musical themes have strengths and differences too, and their strengths lie in different directions; from a composition or improvisation standpoint it's important for organ scholars, especially in this "new world" of post-20th century music, to recognize this and give every theme the chance to grow in the direction best suited to it, including its instrumentation.
The following story illustrates what happens when individual strengths and differences are ignored:
Once upon a time -- the animals of the world decided that they must do something heroic to meet the problems of "a new world."
So they organized a school.
They adopted an activity curriculum consisting of running, climbing, swimming, and flying.
To make it easier to administer that curriculum, ALL the animals took ALL the subjects, and the same level of performance was demanded by all.
The DUCK -- was excellent in swimming, in fact better than his instructor, not bad in flying either, but he was very poor in running.
Since he was slow in running, he had to stay after school and also drop swimming in order to practice running.
This was kept up until his webbed feet were badly worn and he was only average in swimming.
But average was acceptable in school, so nobody worried about that except the Duck.
The RABBIT -- started at the top of the class in running, but had a nervous breakdown because of so much make-up in swimming.
The SQUIRREL -- was excellent in climbing until he developed frustration in the flying class where his teacher made him start from the ground up instead of from the tree-top down.
He developed "charlie horses" from over-exertion and then got a "C" in climbing and a "D" in running.
The EAGLE -- was a problem child from the word "go" and was disciplined severely.
In the climbing class he beat all the others to the top of the tree, but insisted on using his own way to get there.
At the end of the year, an abnormal BAT that could fly exceedingly well -- and also run, climb, and even swim a little -- had the highest average and was valedictorian.
The PRAIRIE DOGS -- stayed out of school altogether and fought the tax levy because the administration would not add digging and burrowing to the curriculum.
The apprenticed their child to a BADGER and later joined the GROUND HOGS and GOPHERS to start a successful private school.
Seriously, there's a moral to this story, for composers:
Musical themes, just like animals and people, have their strengths and weaknesses; when they're well suited to one application they may not lend themselves equally well to another.
They should be carefully chosen with an eye to the task at hand.
It's rather important to bear in mind, in defining a good fugue theme (called the "subject"), what constitutes a poor subject.
Prior to the 20th century composers were in general agreement about this, that is to say, which subjects lend themselves to fugal procedure and which do not.
Composers after 1910 however, were no longer in such agreement and employed an astounding array of new methods and techniques for building their compositions, including contrapuntal pieces like fugues.
Since then composers of the 21st century have made additional contributions to the development of classical music and, since living composers are all wired differently, the answer to this today will depend upon which composer is being asked.
NOTE: Many books consider all who wrote music in the 20th century, beginning around 1910, as contemporary or modern composers, but that sort of label may not be appropriate any more. This author, at least, is not convinced that composers who have been dead more than 70 or 80 years should be referred to as "contemporary" or "modern." Those who have been writing music since the year 2000 should perhaps be in a second category. Significant contributions to classical music have been made by composers of the 21st century such that a division between these 2 centuries of composing seems appropriate, and historians of the future can be expected to place a dividing line between the end of one period and the beginning of the next, just as they have for every era of musical development right back to early music times.
For those 21st century composers who write tonal fugues and work them in baroque common practice style, any subject that's "unsingable" and steeped in poverty in terms of potential for contrapuntal manipulation would be considered a poor, if not pathetic, fugue subject.
This would include subjects with awkward leaps of a tritone, 7ths, 9ths, or larger intervals, subjects with monotonous rhythms or motion (the same series of note durations repeated in every bar, half bar, or quarter bar, mostly in one direction), subjects whose range exceeds the octave (because normal voice ranges would be exceeded when the subject moves to different keys), and subjects with ambiguous tonality (poor sense of key center) lacking a clear beat.
Under these terms a good countersubject, when present (and each succeeding countersubject, if any), will have these same characteristics plus be mutually complimentary to the subject (called the "answer" when repeated) in its rhythm (note durations) and voice motion (melodic outline).