For this reason it inevitably resulted that the sharply defined characters of the phyletic stages were more and more lost as soon as they were transferred from larval stages to stages in embryogenesis. For, in the first place, these sharply defined characters, such as the spines of the Zoæa larva, or the swimming bristles of the 'oars,' or the shape of thorax or abdomen characteristic of certain species, are adapted to a free life, and would be valueless in an embryonic stage; and secondly, in the transference of the free larval stages to embryonic development the greatest possible condensation and abbreviation of the stages must have been striven for, which could only come about by a continual mutual adaptation of the embryonic parts to one another, involving the suppression of everything 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, Zoæa, or Mysis stages in the larval development of the Decapods. I have noticed already that in the water-fleas (Daphnidæ) 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 be 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 precision beyond reasonable limits, and would obscure our insight into the causal connexion between phylogeny and ontogeny, which, as we have seen, undoubtedly exists.