``the fact that life is hierarchical redirects attention to the effect of history on biological phenomena. The major features of genomic organization, of cell architecture, and of organismal ontogeny arose as the products of history-dependent variation at the time of the transition from one unit of selection to another. Units which persist today do so because variants which restrained further interaction between two units of selection arose and fixed the organization of the unit in question at a given state'' [Buss 87] (p.187).
Similarly, Maynard Smith says ``My own view is of a series of historical accidents, subject to engineering constraints on the one hand, and to the conservatism of development on the other'' [Maynard Smith 86] (p.45). The `engineering constraints' arise when there may be only a small number of ways of solving a particular engineering problem faced by a species. By `conservatism of development', Maynard Smith is referring to the fact that ``it seems to be a general feature of evolution that new functions are performed by organs which arise, not de novo, but as modifications of pre-existing organs'' (ibid. p.46).
Finally, Gould's view on the subject is as follows:
``Invariant laws of nature impact the general forms and functions of organisms; they set the channels in which organic design must evolve. But the channels are so broad relative to the details that fascinate us! The physical channels do not specify arthropods, annelids, mollusks, and vertebrates, but, at most, bilaterally symmetrical organisms based upon repeated parts ... When we set our focus upon the level of detail that regulates most common questions about the history of life, contingency dominates and the predictability of general form recedes to an irrelevant background ... Charles Darwin recognized this central distinction between `laws in the background' and `contingency in the details' ...'' [Gould 89] (pp.289-290).