‘It is absurd to talk of one animal being higher than another – we consider those, where the cerebral structure/intellectual faculties most developed, as highest. – A bee doubtless would where the insects were most developed.’ Darwin

‘…And the development of mind, whether of the ‘intellectual’ or ‘instinctive’ type, was obviously not the only way of ranking living things since it left out the vegetable world, and ‘who with the face of the earth covered with the most beautiful savannahs and forests dare to say that intellectuality in the only aim in this world?’…

Nevertheless, something which might reasonably be called progress had clearly happened in the biological world, although in many different dimensions. Exactly what this progression consisted in was not clear, but Darwin was inclined to accept the notion of complexity. Yet he was constitutionally sceptical about the necessity of the process. If complexity had tended to increase in the biological world, then this was a phenomenon that should receive an explanation that was not entirely self-referring. Again the crucial idea is the blindness of the evolutionary process, which responds only to contingency. Darwin realized that in general the contingencies favoured diversification: the very existence of one group of organisms created new niches or another group (as in the simple case of plants creating ipso facto the possibility of animals) and so on indefinitely. The level of complexity of organisms was thus a function of the level of complexity of the organic environment.’

(from ‘A Very Short Introduction to Darwinism’)