I have long been rather dubious about the notion of complexity. Certainly, it is not something that is always desirable. For instance, in theology, as is well known, a central Christian claim is that God is the ultimate simple. In mathematics, simplicity and elegance are highly valued. One thinks for example of the Euler identity: eiπ + 1 = 0. And in science too, there is much to be said for simplicity. Charles Darwin's supporter Thomas Henry Huxley, on being informed about natural selection, is supposed to have exclaimed: “How simple, how stupid not to have thought of that!” More generally in science, the ultimate in scientific achievement is bringing disparate parts of enquiry together under the same hypothesis. This process, known as a “consilience of inductions”, is the paradigmatic exercise in producing simplicity (Whewell, 1840).

However, there is no doubt that the notion of complexity (and related notions of progress) continue to fascinate and absorb our attention. In this discussion, I would like to offer a few remarks about complexity (and progress) as they play out in evolutionary biology. To anticipate, it has long been recognized by historians of evolutionary biology that in the middle of the nineteenth century in Britain there were two competing views of the evolutionary process (Richards, 1987; Ruse, 2013a). One was expressed by Charles Darwin in his Origin of Species (1859), the other by Herbert Spencer in a series of writings, culminating in the 1860s with his First Principles (1862) and his Principles of Biology (1864).