Haeckel was not the only one to create a series of drawings representing embryonic development. Karl E. von Baer and Haeckel both struggled to model one of the most complex problems facing embryologists at the time: the arrangement of general and special characters during development in different species of animals. In relation to developmental timing, von Baer's scheme of development differs from Haeckel's scheme. Von Baer's scheme of development need not be tied to developmental stages defined by particular characters, where recapitulation involves heterochrony. Heterochrony represents a gradual alteration in the original phylogenetic sequence due to embryonic adaptation.