Why was Darwin hesitant to publish his work?

Darwin was hesitant to publish his work because of what others would think of him. Darwin delayed it by two decades and cared very deeply of what others' thought of him. Darwin was most worried about the reaction of the church authorities and his colleagues that were advanced.

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answer; polycythemia is the correct answer;

the correct answer is: b. pantala flavescens, because these organisms would have a high level of genetic variation due to sexual reproduction.  

because involves two organisms (parents) whose genetic material combines, sexual reproduction leads to genetic variation in new generations of offspring. the offspring is an unique organism, with new genome, more protected with the higher ability to adapted. this process is fundamental to evolution.