Shea takes a different approach, arguing that different causal factors have different evolutionary histories. Some causal factors are simply persisting features of the environment (gravity being one example). Others are experienced by the developing organism as a result of histories of selection. Burrows, for example, ensure that eggs and nestlings develop in fairly constant temperature and humidity. But burrows are not naturally selected inheritance mechanisms. They have not come into existence to ensure that a seabird chick resembles its parents. In contrast, some other developmental features are present and act in development because of histories of selection in which the selective advantage is that these mechanisms help ensure parent-offspring similarity. Shea argues that genes, probably epigenetic markings on genes, and perhaps a few other developmental resources are shaped by this form of natural selection. So genes, and perhaps a few other developmental factors, play a distinctive developmental role, even though many other factors are causally necessary (Shea 2011).