THE BIOGENETIC LAW 187 



such apparently simple transformations, the older stage does not, in 

 every ontogeny, precede the more recent one as a preparation for it. 

 though it may be only foi- a brief and transient period. 



It is certain, however, that variations such as the addition of 

 a new stage in ontogeny are undergone, and that this implies the 

 occurrence of something really quite new. Therefore such a new 

 stage can arise only from the germ-plasm, by the duplication, and in 

 part variation, of the determinants of the preceding stage. If, for 

 instance, the body of a Crustacean be lengthened by a segment, this 

 must be due to a process of this kind, and in such a case it is 

 intelligible enough that the new segment can be formed in the 

 ontogeny only after the development of the older preceding one, for 

 its determinants come from that, and are from the beginning so 

 arranged that they are only liberated to activity by the formation 

 of the preceding segment. 



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, tlntil finally they appeared dinultaneously : the 

 determinants of the segments n, n+i,n + 'Z. . . . n + x varied in regard 

 to their liberating stimuli, and were roused to activity no longer 

 successively, but simultaneously, in the cell complexes controlled Ijj^ 

 them. We have thus recapitulation, but with abridgement and com- 

 pression, of the phyletic stages in the ontogeny. Thus in the nauj^lius 

 of Leptodora we see the rudiments of five of the pairs of legs of the 

 subsequent thorax (Fig. iii^ IV-VIII), and in the Zosea 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 Caprellidas among the 

 Amphipod Crustaceans the whole abdomen of from six to seven seg- 

 ments has degenerated to a narrow, rudimentary structure. In such 

 cases the gi-adual degeneration of the relative determinants has pre- 

 ceded step for step the degeneration of the part itself, and when tliis 

 is complete the ontogeny shows nothing of what was previously 

 present, and so we may speak of a ' falsification ' of the phylogeny. 



