What stopped Darwin from discovering the laws of inheritance?

At Access Research Network, David Tyler asks, what stopped Darwin from discovering the laws of inheritance?":

In a helpful analysis of the issues, Jonathan Howard of the University of Cologne suggests that it is legitimate to ask why Darwin did not reach a satisfying conclusion: "The solution, at least to the inheritance problem, was apparently easily amenable to an experimental approach with materials that were daily to hand. Furthermore the time was ripe in the middle of the 19th century, with many breeders interested in the problem for commercial as well as scientific reasons. And after all, Mendel solved the logic of inheritance in his own backyard in the monastery at Brno with no more technology than Darwin had at his disposal in his garden at Down House. Why couldn't Darwin have done it too?"

Howard looks carefully at Darwin's writings on the subject, and finds he was highly selective in his interests. He sought to document the small variations that he thought could accumulate and lead eventually to speciation. His writings reflect his commitment to this concept: "the selection of infinitesimal varieties", "differences absolutely inappreciable to the naked eye", "the accumulation of infinitesimally small inherited modifications". Howard makes this comment on Darwin's 1876 book The Effects of Cross and Self-Fertilisation in the Vegetable Kingdom: "His overriding purpose was to establish that progeny produced by self-fertilization are less thrifty than the products of cross-fertilization. He rightly concentrated his analysis not on unit characters, but on the quantitative characters that fitted better with his concept of differential fitness, the attributes that, by their infinitesimal differences, determine life and death in the wild. So Darwin counted seeds, weighed and measured them, planted them and looked for their vitality. He measured growth and general thriftiness in his self-fertilized and cross-fertilized progeny. Everything he measured was a quantitative variable that under these simple experimental conditions could yield no information about inheritance at all. His experiments are overwhelming in scale and scope; they established the point that he wanted to establish beyond all doubt, but they contributed nothing relevant to our understanding of the underlying logic of inheritance."

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