The history of life on Earth seems to show a clear tren; for example, it seems obvious that there is a trend towards increasing complexity in living organisms. More recent organisms, such as mammals, appear to be much more complex than older organisms, such as bacteria. However, there are theoretical and empirical problems with this claim. From a theoretical perspective, it appears that there is no reason to expect evolution to result in any largest-scale trends, although small-scale trends, limited in time and space, are expected (Gould, 1997). From an empirical perspective, it is difficult to measure complexity and, when it has been measured, the evidence does not support a largest-scale trend (McShea, 1996).