Historians have remarked that here Darwin anticipated the modern concept of an ecological niche. He did not suggest that every favourable variation must be selected, nor that the favoured animals were better or higher, but merely more adapted to their surroundings. Darwin proposes sexual selection, driven by competition between males for mates, to explain sexually dimorphic features such as lion manes, deer antlers, peacock tails, bird songs, and the bright plumage of some male birds. He analysed sexual selection more fully in The Descent of Man, and Selection in Relation to Sex (1871). Natural selection was expected to work very slowly in forming new species, but given the effectiveness of artificial selection, he could "see no limit to the amount of change, to the beauty and infinite complexity of the coadaptations between all organic beings, one with another and with their physical conditions of life, which may be effected in the long course of time by nature's power of selection". Using a tree diagram and calculations, he indicates the "divergence of character" from original species into new species and genera. He describes branches falling off as extinction occurred, while new branches formed in "the great Tree of life ... with its ever branching and beautiful ramifications". Variation and heredity In Darwin's time there was no agreed-upon model of heredity; in Chapter I Darwin admitted, "The laws governing inheritance are quite unknown." He accepted a version of the inheritance of acquired characteristics (which after Darwin's death came to be called Lamarckism), and Chapter V discusses what he called the effects of use and disuse; he wrote that he thought "there can be little doubt that use in our domestic animals strengthens and enlarges certain parts, and disuse diminishes them; and that such modifications are inherited", and that this also applied in nature. Darwin stated that some changes that were commonly attributed to use and disuse, such as the loss of functional wings in some island-dwelling insects, might be produced by natural selection. In later editions of Origin, Darwin expanded the role attributed to the inheritance of acquired characteristics. Darwin also admitted ignorance of the source of inheritable variations, but speculated they might be produced by environmental factors. However, one thing was clear: whatever the exact nature and causes of new variations, Darwin knew from observation and experiment that breeders were able to select such variations and produce huge differences in many