This tremendous complexity allows the parasite to constantly change its surface and thus evade the immune system through antigenic variation. More generally, the growth of complexity may be driven by the co-evolution between an organism and the ecosystem of predators, prey and parasites to which it tries to stay adapted: as any of these become more complex in order to cope better with the diversity of threats offered by the ecosystem formed by the others, the others too will have to adapt by becoming more complex, thus triggering an ongoing evolutionary arms race towards more complexity. This trend may be reinforced by the fact that ecosystems themselves tend to become more complex over time, as species diversity increases, together with the linkages or dependencies between species. If evolution possessed an active trend toward complexity (orthogenesis), as was widely believed in the 19th century, then we would expect to see an active trend of increase over time in the most common value (the mode) of complexity among organisms. However, an increase in complexity can also be explained through a passive process. Assuming unbiased random changes of complexity and the existence of a minimum complexity leads to an increase over time of the average complexity of the biosphere.This involves an increase in variance, but the mode does not change. The trend towards the creation of some organisms with higher complexity over time exists, but it involves increasingly small percentages of living things. In this hypothesis, any appearance of evolution acting with an intrinsic direction towards increasingly complex organisms is a result of people concentrating on the small number of large, complex organisms that inhabit the right-hand tail of the complexity distribution and ignoring simpler and much more common organisms. This passive model predicts that the majority of species are microscopic prokaryotes, which is supported by estimates of 106 to 109 extant prokaryotes compared to diversity estimates of 106 to 3·106 for eukaryotes. Consequently, in this view, microscopic life dominates Earth, and large organisms only appear more diverse due to sampling bias. Genome complexity has generally increased since the beginning of the life on Earth. Some computer models have suggested that the generation of complex organisms is an inescapable feature of evolution. Proteins tend to become more hydrophobic over time, and to have their hydrophobic amino acids more interspersed along the primary sequence. Increases in body size over time are sometimes seen in what is known as Cope's rule. Recently work in evolution theory has proposed that by relaxing selection pressure, which typically acts to streamline genomes, the complexity of an organism increases by a process called constructive neutral evolution.