Another of the chief lines of evidence for the truth of the evolution theory is based on the study of embryology, and this also was followed with great vigour by the zoologists of the last thirty years of the nineteenth century. It is found that in many instances animals recapitulate in their early development the stages through which their ancestors passed in the course of evolution. Land Vertebrates, including man, have in their early embryonic life gill-clefts, heart and circulation, and in some respects skeleton and other organs of the type found in fishes, and this can only be explained on the assumption that they are descended from aquatic fish-like ancestors. On the basis of such facts as these, the theory was formulated that every animal recapitulates in ontogeny (development) the stages passed through in its phylogeny (evolution), and great hopes were founded upon this principle of discovering the systematic position and evolutionary history of isolated and aberrant forms. In many cases the search has led to brilliant results, but, as in the case of palaeontology, in many others the light that was hoped for has not been forthcoming. For it soon became evident that the majority of animals show adaptation to their environment not only in their adult stages but also in their larval or embryonic period, and these adaptations have led to modifications of the course of development which are often so great as to mask, or obscure altogether, the ancestral structure which may once have existed. Although, therefore, the results of embryological research have provided most convincing proof of the truth of the theory of evolution in general, they have not completely justified the hopes of the early embryologists that by this method all the outstanding phylogenetic problems might be solved.