What did Darwin mean by his famous "tangled bank" analogy?

The tangled bank illustrates the diversity of life that we can see all around us: It is interesting to contemplate an entangled bank, clothed with many plants of many kinds, with birds singing on the bushes, with various insects flitting about, and with worms crawling through the damp earth, and to reflect that these elaborately constructed forms, so different from each other, and dependent on each other in so complex a manner, have all been produced by laws acting around us. The “tangled bank” is used to illustrate how complexity can arise from the interaction of a few simple rules for natural selection. This is actually the final grand metaphor in the Origin, and in it he tries to provide readers with a vision of life’s diversity, underpinned by the common relationships of species.