question: What are Darwin 's thoughts on producing hybrids of related species ?
answer: the viability and fertility of the hybrids , varied greatly , especially among plants
turker_answer: Darwin 's thought the viability and fertility of the hybrids , varied greatly , especially among plants
rule-based: Darwin 's thoughts on producing hybrids of related species are the viability and fertility of the hybrids , varied greatly , especially among plants .