172 THE EVOLUTION THEORY 



thing superfluous. Otherwise the transference of the free stages to 

 the embryogenesis would have brought no advantage, but rather 

 a most prejudicial protracting of the development. 



We must not, therefore, expect to find the stages of the 

 phylogeny occurring unaltered in every ontogeny in the way we have 

 found the nauplius, Zo^a, or Mysis stages in the larval development of 

 the Decapods. I have noticed already that in the water-fleas 

 (Daphnidge) and other Crustaceans without metamorphosis the 

 nauplius stage is still passed through, but within the egg, and as an 

 embryonic stage, and this is quite true, but nevertheless it would 

 hardly do to liberate a nauplius like this from its shell and place it in 

 the water, for the influence of the water upon the delicate embryonic 

 cells of its body would soon cause it to swell, and would destroy 

 it utterly. And, even apart from this, it has no hard and resistant 

 chitinous covering, no fully-developed appendages, but only the 

 stump-like blunt beginnings of these without swimming-bristles and 

 without muscles capable of function, so that it could not even move. 

 Nevertheless it is a nauplius with all its typical distinctive characters, 

 only it is not a perfect nauplius capable of life, but rather a ' schema ' 

 of one, which must b& retained in the embryogenesis that it may give 

 rise to the later stages. 



Shall we therefore say that the statement that phylogeny 

 repeats itself in ontogeny is false, that the nauplius stage within the 

 embryo is not a true nauplius at all ? That would be pushing pre- 

 cision beyond reasonable limits, and would obscure our insight into 

 the causal connexion between phylogeny and ontogeny, which, as we 

 have seen, undoubtedly exists. 



A few years after the appearance of Fritz Miiller's work Filr 

 Darwin, Haeckel elaborated Miiller's idea, and applied it in a much 

 more comprehensive manner. He formulated it under the name of 

 ' the fundamental biogenetic law,' and then he used this ' law ' to 

 deduce from the ontogeny of animals, and more particularly of Man, 

 the paths of evolution along which our modern species have passed in 

 the course of the earth's history. In doing so the greatest caution 

 was necessary, since ontogeny is not an actual unaltered recapitulation 

 of the phylogeny, but an ' abridged ' and in most cases- — in my own 

 belief, in all cases — a greatly modified recapitulation. Therefore we 

 cannot simply accept each ontogenetic stage as an ancestral stage, but 

 must take into consideration all the facts supplied to us by other 

 departments of biological inquiry which afford help in the decision of 

 such questions, especially those brought to light by comparative 

 morphology and by the whole range of comparative embryology. 



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