Complex sentence: Meiners' classification was not grounded on any scientific criteria, and it was Blumenbach who gave it scientific credibility and a wider audience, by grounding it in the new quantitative method of craniology.
Split into simpler sentences: Meiners' classification was not grounded on any scientific criteria. It was Blumenbach who gave it scientific credibility and a wider audience, by grounding it in the new quantitative method of craniology.