There are conditions satisfied by successful analogical arguments which Darwin’s argument satisfies. Darwin first establishes that breeding practices are an analogical model of the struggle for existence in the wild: just as humans discriminate in favour of animals and plants with desirable traits, so the struggle for existence discriminates in favour of creatures with traits best enabling them to cope with that struggle. Domestic breeding creates new varieties because it is systematic – there will be a tendency always to discriminate in favour of the same set of traits. The struggle for existence will have the same systematic tendency to favour certain traits at the expense of others. Therefore it is possible for it also to create new varieties. Darwin now alternates the analogy and its proportionality: if natural selection (NS) is to new wild varieties as artificial selection (AS) is to new domesticated varieties, then NS is to AS as new wild varieties are to new domesticated varieties. Thus if NS is a massively more efficient selector than AS, and the greater the cause, the greater the effect, then, a fortiori, NS should produce not only new varieties but new species.