 In the 1950’s, Croizat proposed an alternative to the Darwin-Wallace theory which rejects their presupposition of geographical stability. Croizat argues that tectonic change, not dispersal, is the principal causal mechanism which underlies Buffon’s Law. Forces such as continental drift, the submerging of ocean floors, and the formation of mountain ranges have acted within the time frame of evolutionary history to create natural barriers between species where at previous times there were none. Croizat’s theory was the sophisticated culmination of a theoretical tradition which stretched back to the late 17th Century. Followers of this so-called “extensionist” tradition had postulated the existence of ancient land bridges to account for anomalies in the geographical distribution of plants and  animals.[13]