5l6 THE HISTORY OF BIOLOGY 



symmetry. He gives instead a special account of the descent of man, which 

 Haeckel regarded all along as the centre point of the theory of evolution 

 and of all science in general. This problem likewise represents, as the title 

 indicates, the subject of his Ant hrofo genie, a work of far greater significance 

 than the History of the Creation and certainly the one in which Haeckel has 

 set forth h'is most brilliant and most important ideas — those of his that 

 most deeply affected the development of biology. The intention of the work 

 is to give a comprehensive idea of the origin of man, based on the evidence 

 of morphology, embryology, and palaeontology. Haeckel definitely takes as 

 his starting-point his well-known "biogenetical principle": that the on- 

 togeny not only of man, but also of every living creature is a recapitulation 

 of its phylogeny; "the development of the embryo is an abstract of the his- 

 tory of the genus." This idea in itself is not new; as we have seen, it had 

 already been propounded by Meckel, and Darwin gave it an important place, 

 although it was formulated in summary fashion, in his Origin of Species. It 

 was then taken up and further elaborated by Fritz Muller (1811-97), one 

 of the more peculiar representatives of biology during last century. 



Fritz Muller on the development of crayfish 

 Born in Germany, Fritz Muller had studied medicine — among other things, 

 biology under J. Muller — but afterwards went out to Brazil, where he re- 

 mained for the rest of his life in various occupations and with varying for- 

 tunes. Having from the very beginning been entirely won over to Darwin's 

 theory, he resolved to prove it by applying it in detail to a suitable animal 

 group, for which purpose he chose Crustacea, which in his adopted country 

 exist in a multitude of forms. He paid special attention to the different types 

 of development to be found in closer related forms within this class: the 

 river crayfish creeps out of the e§,g like its parents; the crabs have one or 

 two larval forms, while the prawn has many — a nauplius stage similar to 

 the larvas of the lowest Crustacea described under that name, a 7j)ea stage, 

 like that of the crabs, and a my sis stage, like the perfected form of the schiz- 

 opod crayfish. Various other Crustacea likewise possess peculiar metamor- 

 phoses, especially the strangely formed parasite crayfish, whose early stages 

 resemble those of the independently living Crustacea. All these facts, espe- 

 cially the fact that the larvas of certain higher Crustacea resemble the fully 

 grown individuals of lower Crustacea, convinced Fritz Muller that the evo- 

 lution of the individual is a "historical document," which is sometimes 

 effaced, owing to the development's striking into a more and more direct 

 path from the tgg to the fully grown creature, and which is sometimes 

 "counterfeited, owing to the struggle for existence that the independently 

 existing larva; have to maintain." A case such as that of the prawn he re- 

 gards as typical; the prawn's ancestors in past ages possessed the form that 

 its larva; now possess, and that, too, in the same sequence as that in which 



