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Google scholar has a full-text index of nearly all research papers. You can use it to get counts for arbitrary phrases. I've been using this to measure popularity of things in science. For example, here's the popularity of Greek letters used in equations 1/3

This implies Human Ethics have changed over time. When possible, testing has switched from 'ape' to 'mouse'. Would be interesting (if possible) to see testing count on 'homo sapiens' as well.