His great contribution was to establish scrupulous standards in the acceptance of evidence which for many years were not to be surpassed. Lesson says that birds of paradise, so remarkable for their sexual differences , are polygamous, but Mr. Wallace doubts whether he had sufficient evidence. Hence it appears that amongst birds there often exists a close relation between polygamy and the development of strongly-marked sexual differences . Here they fight very much like gamecocks, seizing each other with their beaks and striking with their wings. The ruff of feathers, however, from its varied and rich colours probably serves in chief part as an ornament.
Naturalists are much divided with respect to the object of the singing of birds.
I will now discuss in some detail the degree of fertility of certain hybrids in successive generations. We will now consider the variability of the nesting instinct. The incalculable host of parasites which pass their whole lives on or in the bodies of other animals do not here especially concern us. Hence it might well be a great advantage to a Cuckoo to lay her eggs in other birds nests. Chaga's disease, in which parasites often infiltrate the central nervous system, has no effective treatment, is eventually fatal, and is quite widespread.