  First, notice the use of the language of ‘tendencies’ and ‘frequencies’ in the above principles. Privately, Darwin learned, Herschel had referred to his theory as ‘the Law of higgledy-piggledy’, presumably a reference to the large element played in its key principles by chance and probability. Darwin’s theory is, as we would say today, a ‘statistical’ theory. One cannot say that every individual with favorable variation v will survive or will leave more offspring than individuals without it; one cannot say that no environment will ever support all of the offspring produced in a given generation, and thus that there must always be a competitive struggle. These are things that tend to happen due to clearly articulated causes, and this allows us to make accurate predictions about trends, at the level of populations, but not to make absolute claims about what must happen in each and every case. Only well after Herschel’s time did philosophers of science become comfortable with the idea of a theory of this sort, and  the proper philosophical understanding of such explanations is still debated.