160 THE EVOLUTION THEORY 



stages, from polyp and worm up to insect and mollusc. But von Baer 

 afterwards showed that such resemblances are never between different 

 types, but only between representatives of the same general type, 

 e. g. that of Vertebrata ; and Johannes Mliller maintained, from the 

 standpoint of 'the old Creation theory, that an ' expression of the most 

 general and simple plan of the Vertebrates ' recurred in the develop- 

 ment of higher Vertebrates, giving as an instance that, at a certain 

 stage of embryogenesis, even in Man, gill-arches were laid down and 

 were subsequently absorbed. But why this ' plan ' should have been 

 carried out where it was afterwards to be departed from remained 

 quite unintelligible. 



An answer to this question only became possible with the revival 

 of the. Theory of Descent, and the first to throw light in this direction 

 was Fritz Miiller, who, in his work Filr Dariuin, published in 1864, 

 interpreted the developmental history of the individual, ' the ontogeny,' 

 as a shortened and simplified repetition, a recapitulation, so to speak, 

 of the racial history of the species, the ' phylogeny.' But at the same 

 time he recognized quite clearly — what indeed was plain to all eyes — 

 that the ' racial history ' cannot be simply read out of the ' germinal 

 history,' but that the phylogeny is often ' blurred,' on the one hand 

 by the fusing and shortening of its stages, since development is 

 always ' striking out ' a more direct course from the egg to the perfect 

 animal, while, on the other hand, it is frequently ' falsified ' by the 

 struggle for existence which the free-living larvee have to maintain. 



For the establishment of these views Fritz Miiller relied chiefly 

 upon larvge, and in particular upon those of Crustaceans, and the 

 facts, which were in part new and in part interpreted in a new 

 manner, were so striking that it was impossible to deny their 

 importance. In particular, he drew attention to the fact that in 

 several of the lower orders of Crustaceans the most diverse species 

 have a similar form when they leave the egg, all of them being small, 

 unsegmented larvsg, with a frontal eye and a helmet-like upper lip^ 

 and with three pairs of appendages, the two posterior pairs being two- 

 branched swimming-legs beset with bristles. In the size and form 

 of the body, and especially of the chitinous carapace, these larvae 

 differ in the various systematic groups ; thus, for instance, the larvae 

 of the Copepods are simply oval, while those of the Cirrhipedes are 

 produced anteriorly into two horn-like processes, and so on, but in 

 essentials they are all alike, and for a long time these larval forms 

 had been distinguished by the special name of ' Nauplius ' (Fig. 109). 



The development of the perfect animal begins with the longitu- , 

 dinal growth of the Nauplius ; the posterior end lengthens and 



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