is an apt qualification of the unification process, it is subject to the more general mechanisms and principles of evolution just described. First, some general observations that stem from the previous survey are appropriate for the purpose of this contribution. Then, I will elaborate some points in more detail First, the question should be put to what extent the three darwinist requirements for a survival of the fittest are also applicable to European private law. In an evolutionary theory of European unification, the various national rules to solve similar problems may be regarded as the necessary variety of species. This variety has come about through differentiation that started from one" tree of law"(some general concept of fairness from which the various rules originated). The second requirement as to the variation in fitness is met as well as long as it is presumed that not all the legal rules are as"fit as others to carry out their task. Some rules may have been eliminated by the environment in which they had to operate; others may have survived because of their ability to adjust themselves to changing circumstances. The third requirement (inheritability of characteristics) is met if legal institutions are looked at as the"genes"of a legal system: the content of these institutions may differ as the identity of the institution as such remains the same Gust like in economics, routines establish a stable identity of a firm). Just like new routines of firms are seldom entirely new, but most of the time combinations of old ones that guarantee that the specific identities of these firms are maintained, the institutions that program the behaviour of the rules in response to the changing environment, maintain their specific identities as well This idea calls for an application in the field of private law(section 5.2) Secondly, evolutionary theory enlightens us regarding the possibility of the best rule surviving in a"struggle for life". This is definitely not a straightforward mechanism. To predict which rules survive and which do not, one cannot just take efficiency or any other mono- explaining mechanism(the"simple model )as a key-concept. Two different sorts of barriers to the emergence of the best legal rule should be taken into account. First, historically, the rule that has emerged may have been best adapted to the environment in which it had to function in the past. This has for a consequence that some rules may have been responsive to that past environment, at the same time eliminating the rules that were not adaptive in those days, but may have been the best rule for present times, if it had not been for their elimination. Selection on the market of legal rules does in this sense not produce the best available rules. Second, there is a future oriented aspect of this approach as well. Even if one is able to reinvent the rule that disappeared(and legal history can play an important role in doing so), it may be too costly to have that other rule prevail over the one we have become accustomed to. In this case of path dependence(the future development is affected by the path it has set out in the past),an quilibrium cannot evolve. Accidents" may thus be just as important to explain the past and the future development of law. I will elaborate this idea with a view to the harmonisation process in different areas of private law in section 5.3 Thirdly, it is fertile to look at legal rules as having a desire to reproduce themselves. This analysis may explain why it is that over time identical legal rules are often used for different goals This"Funktionswandel"of a rule may indeed happen more frequently than the clear-cut elimination of a rule. As we saw that organisms fit themselves into"niches of viability offered by their environments, legal rules want to survive as well in a changing society. It then is only because of the use of the same terminology or the embedment within the same institution that a stable identity remains(this point is related to the one discussed in section 5.2). Legal transplants Vromen, supra note 44, at 53 88 is an apt qualification of the unification process, it is subject to the more general mechanisms and principles of evolution just described. First, some general observations that stem from the previous survey are appropriate for the purpose of this contribution. Then, I will elaborate some points in more detail. First, the question should be put to what extent the three Darwinist requirements for a survival of the fittest are also applicable to European private law. In an evolutionary theory of European unification, the various national rules to solve similar problems may be regarded as the necessary variety of species. This variety has come about through differentiation that started from one “tree of law” (some general concept of fairness from which the various rules originated). The second requirement as to the variation in fitness is met as well as long as it is presumed that not all the legal rules are as “fit” as others to carry out their task. Some rules may have been eliminated by the environment in which they had to operate; others may have survived because of their ability to adjust themselves to changing circumstances. The third requirement (inheritability of characteristics) is met if legal institutions are looked at as the “genes” of a legal system: the content of these institutions may differ as the identity of the institution as such remains the same (just like in economics, routines establish a stable identity of a firm). Just like new routines of firms are seldom entirely new, but most of the time combinations of old ones that guarantee that the specific identities of these firms are maintained,61 the institutions that program the behaviour of the rules in response to the changing environment, maintain their specific identities as well. This idea calls for an application in the field of private law (section 5.2). Secondly, evolutionary theory enlightens us regarding the possibility of the best rule surviving in a “struggle for life”. This is definitely not a straightforward mechanism. To predict which rules survive and which do not, one cannot just take efficiency or any other mono￾explaining mechanism (the “simple model”) as a key-concept. Two different sorts of barriers to the emergence of the best legal rule should be taken into account. First, historically, the rule that has emerged may have been best adapted to the environment in which it had to function in the past. This has for a consequence that some rules may have been responsive to that past environment, at the same time eliminating the rules that were not adaptive in those days, but may have been the best rule for present times, if it had not been for their elimination. Selection on the market of legal rules does in this sense not produce the best available rules. Second, there is a future oriented aspect of this approach as well. Even if one is able to “reinvent” the rule that disappeared (and legal history can play an important role in doing so), it may be too costly to have that other rule prevail over the one we have become accustomed to. In this case of path dependence (the future development is affected by the path it has set out in the past), an equilibrium cannot evolve. “Accidents” may thus be just as important to explain the past and the future development of law. I will elaborate this idea with a view to the harmonisation process in different areas of private law in section 5.3. Thirdly, it is fertile to look at legal rules as having a desire to reproduce themselves. This analysis may explain why it is that over time identical legal rules are often used for different goals. This “Funktionswandel” of a rule may indeed happen more frequently than the clear-cut elimination of a rule. As we saw that organisms fit themselves into “niches of viability offered by their environments”, legal rules want to survive as well in a changing society. It then is only because of the use of the same terminology or the embedment within the same institution that a stable identity remains (this point is related to the one discussed in section 5.2). Legal transplants 61 Vromen, supra note 44, at 53