The Creation-Evolution Encyclopedia gives the quote:
"It is probably fair to estimate the frequency of a majority of mutations, in higher organisms, between one in ten thousand and one in a million per gene per generation."Dr. Ayala is being quoted to support the idea that mutations pretty well never happen. However, a human cell contains 30,000 to 40,000 genes. So, in fact, Dr. Ayala was saying that a given human might have about four mutations. Or, with the other number, a given human might have one chance in thirty of having a mutation.
Teleological Explanations in Evolutionary Biology, in Philosophy of Science, March 1970, p.3
I didn't bother looking that quote up, because in genetics, a quote from 1970 is obsolete. Recent studies show that one in ten thousand is about right.
There was a second quote:
"Although mutations is the ultimate source of all genetic variation, it is a relatively rare event."I did look this quote up. The article used the word "rare" because it was talking about something even more common. You inherit over 30,000 genes from your mother, and over 30,000 genes from your father. The resulting mix-and-match of variant genes ("alleles") makes mutation seem comparatively rare.
Mechanism of Evolution, Scientific American, September 1978, p. 63.