Why is the diminished scale 'artificial'?
In the sense of the quote you gave, it is artificial because it was not constructed from the overtone series.
What is an overtone series?
http://en.wikipedia.org/wiki/Harmonic_series_%28music%29 explains it very well.
How is a major scale derived from it?
There isn't a direct step by step method. It is easier to justify a pentatonic scale from the harmonic series than anything else. If our western major scale more directly followed the harmonic series it would have a # 4th, much like the lydian scale.
Harmonic series for C
C-C3-G3-C4-E4 G4-B-Flat4*-C5-D5 E5-F-Sharp5*-G5-A5*-B Flat5*-B5-C6.
Now the corresponding C major scale is:
C D E F G A B C
And taken literally the note collection above is:
C D E F# G A Bb B C
If you investigate 18th century counterpoint, you will learn why we do not use this scale, or why the current set of scales is a product of contrapuntal thinking. Often, ethnic, or traditional music will use scales or note sequences more closely following the above. Also, if you are interested in Jazz check out the "lydian chromatic theory" by George Russell.
Long story short:
F# is a no no. Causes a tritone.
Bb is a no no. Causes a tritone.
Now, allow me to deconstruct the author's quote for you:
The diminished scale is artificial in the sense that it is not derived from the overtone series, as is the major scale, and has no particular ethnic origin, as does the melodic minor scale, which has an Eastern European ancestry.
Sadly, music instruction textbooks are not always proofread by ethnomusicologists. Sometimes knowledge of the origins of musical constructs (for performers) is more passed down than maintained via rigorous inquiry.
The diminished scale is artificial in the sense that it is not derived from the overtone series
This is a confusing statement for multiple reasons. First, technically any scale can be justified by the harmonic series, since it producing a series of all possible notes, including thousands of microtones.
as is the major scale
Actually the history of the major scale is far, far more complicated than this and diverges from the "harmonic" series justification often. Like I said, nearly any scale can be justified by the harmonic series. Our major scale has as much to do with arguments over proper cadences in church music as it does the harmonic series. In that sense it is just as "constructed" as the diminished scale.
and has no particular ethnic origin
This is just wrong:
Diminished scales have existed for as long as diatonic scales, in various cultures around the world. They were also used widely by the romantics. Slonimsky wrote extensively about the scale. At any rate it goes way back, and has been used in Western classical music since at least 1823.
which has an Eastern European ancestry.
Also completely wrong. The melodic minor scale was, again, created by composers in Western Europe for properly resolving cadences. It's history and usage, and justification can be traced back to 18th century counterpoint practices throughout western Europe in the 18th century. Perhaps he just thinks it has a vaguely eastern sound?