 Thomas Huxley would make extensive use of both the horse and bird fossils in his advocacy of evolution.Acceptance of evolution occurred rapidly in scientific circles, but acceptance of Darwin's proposed mechanism of natural selection as the driving force behind it was much less universal.In particular some paleontologists such as Edward Drinker Cope and Henry Fairfield Osborn preferred alternatives such as neo-Lamarckism, the inheritance of characteristics acquired during life, and orthogenesis, an innate drive to change in a particular direction, to explain what they perceived as linear trends in evolution.