The degree of systematic relationship is thus the degree of genealogical affinity. It follows that the natural system of classification may be converted straightway into a genealogical tree, and this is actually what Haeckel does in the _General Morphology_. The genealogical trees depicted in the second volume (plates i.-viii.) are nothing more than graphic representations of the ordinary systematic relationships of organisms, with a few hypothetical ancestral groups or forms thrown in to give the whole a genealogical turn. If the genealogical tree is truly represented by the natural system, it would seem that for each genus a single ancestral form must be postulated, for each group of genera a single more primitive form, and so in general for each of the higher classificatory categories, right up to the phylum. Species of one genus must be descended from a generic ancestral form, genera of one family from a single family _Urform_, and so on for the higher categories. This consequence was explicitly recognised by Haeckel. "Genera and families," he writes, "as the next highest systematic grades, are extinct species which have resolved themselves into a divergent bunch of forms (_Formenbueschel_)" (ii., p. 420). The archetype of the genus, family, order, class and phylum was thus conceived to have had at some past time a real existence. The natural system of classification is based upon a proper appreciation of the distinction between homological and analogical characters. Haeckel, following Darwin, naturally interprets the former as due to inheritance, the latter as due to adaptation, using these words, we may note, in their accepted meaning and not in the abstract empty sense he had previously attributed to them.[370] Similarly the "type of organisation," in von Baer's sense, was due to heredity, the "grade of differentiation" to adaptation. So far Haeckel merely emphasised what Darwin had already said in the _Origin of Species_. But by his statement of the "biogenetic law," and particularly by the clever use he made of it, Haeckel went a step beyond Darwin, and exercised perhaps a more direct influence upon evolutionary morphology than Darwin himself. Haeckel was not the original discoverer of the law of recapitulation. It happened that a few years before the publication of Haeckel's _General Morphology_, a German doctor, Fritz Mueller by name, stationed in Brazil, had been working on the development of Crustacea under the direct inspiration of Darwin's theory, and had published in 1864 a book[371] in which he showed that individual development gave a clue to ancestral history.