Born in 1822, one of the finest pioneers in zoological work, Fritz Müller had wished to become a higher teacher, but had abandoned his plan on account of the oath that had to be taken by every servant of the State. In 1849 he wrote to the Ministry requesting that he might be allowed to dispense with the formula “So help me God, through Jesus Christ.” Meeting with a refusal, he went to South America, and began a solitary life as a student in the primitive forest, and sought to accumulate valuable zoological material. Darwin called him “the king of observers.” In 1864 he published an essay of ninety-four pages with the title _For Darwin_. He revived and improved the old idea of Oken’s and made fresh contributions to the natural history of the crustacea that were literally stupefying. We may say that the point that he believed he had established, in virtue of the law, in regard to the genealogical tree of the crustacea, was afterwards, with apparent justice, called into question, even by supporters of the law such as Arnold Lang. That, however, did not diminish the extent of his influence at the time. Haeckel has generously acknowledged how strongly he felt that influence himself. Nevertheless all that has been said about Haeckel’s priority in fully applying and shaping the law, and in its final formulation, is perfectly correct. When Haeckel had massed his material he had first to create the necessary terms for arranging it distinctly. In the language of the old legend, he called the day day, and the night. To the story of ancestral development, or the evolution of the stem, he gave the name of _phylogeny_, or stem-history (_phylon_ = stem). The word circulates very widely to-day. The story of the development of the individual until it reaches maturity was then called _ontogeny_ (_on_ = being), which coincides generally with embryology (though it may also include the growth of the child). The law then ran: Ontogeny is an abbreviated and frequently disarranged epitome of phylogeny. Special attention was drawn to the qualifications “abbreviated” and “disarranged.” Here again two fresh names were invented. In so far as the embryonic development is a true recapitulation of the stem-history, it is called _palingenesis_, or repetition of the ancestral traits. When the development is altered by new adaptations it is called _cenogenesis_, “foreign” or “disturbing” development.