Now, if in the course of the phylogeny numerous new segments were added to the body of the Crustacean, the ontogeny would be materially prolonged, and condensation would become necessary in the interests of species-preservation. To bring this condensation about, whole series of segments which were added successively in the phylogeny succeeded each other with gradually increasing rapidity in the ontogeny, until finally they appeared _simultaneously_: the determinants of the segments _n_, _n_ + 1, _n_ + 2, ... _n_ + _x_ varied in regard to their liberating stimuli, and were roused to activity no longer successively, but simultaneously, in the cell complexes controlled by them. We have thus recapitulation, but with abridgement and compression, of the phyletic stages in the ontogeny. Thus in the nauplius of _Leptodora_ we see the rudiments of five of the pairs of legs of the subsequent thorax (Fig. 111, _IV-VIII_), and in the Zoæa larva the rudiments of six thoracic legs may be seen behind the already developed swimming-leg (Fig. 114, _VI-XIII_). But in the course of the phylogeny a segment may also become superfluous, and we know that it then degenerates and is ultimately eliminated altogether. Thus in a parasitic Isopod, which lives within other Crustaceans, a segment of the thorax is wanting in the relatively well-developed larva, and in the Caprellidæ among the Amphipod Crustaceans the whole abdomen of from six to seven segments has degenerated to a narrow, rudimentary structure. In such cases the gradual degeneration of the relative determinants has preceded step for step the degeneration of the part itself, and when this is complete the ontogeny shows nothing of what was previously present, and so we may speak of a 'falsification' of the phylogeny. But that the complete disappearance of the determinants only comes about with extreme slowness, so that whole geological periods are sometimes not enough for its accomplishment, we have already learnt from our study of rudimentary organs, instances of which can be demonstrated in every higher animal, bearing witness to the presence of the relevant organs or structures in the ancestors of the species.