Like many other Victorian upper-class families and most of his relatives, Charles Darwin married within the clan. He married his cousin, Emma. Six years after his wife bore her last child, Darwin began to wonder about the dangers of inbreeding since a cousin of his, Francis Galton, pointed out the dangers of marrying within the clan. He soon became anxious about the health issues in his own children. One of his daughters, Henrietta, inherited stress-induced vomiting from him, his son Leonard was “rather slow and backward”, Horace had attacks of shuddering, sobbing, and suffered from semi-convulsive movements. George had an irregular pulse, and his much-loved daughter, Annie, died at 10.
Darwin started experimenting on plants to find out more about inbreeding. He forced the plants to self-pollinate and discovered that the next generations fared badly. The inbred plants of next generations became small, less vigorous and weaker than those that cross-pollinated, leading him to realize that “cross-fertilization is generally beneficial, and self-fertilization injurious.”