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 for 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 tliis implies the 

 occurrence of something really quite new. Therefore sucli 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, tliis 

 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 tlie 

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

 its determinants come from that, and are from the beginninc>- 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 ontogenj^ 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 tlie 

 phylogeny succeeded each other with gradually increasing rapidity 

 in the ontogeny, until finally they appeared sionulta neoiislij : the 

 determinants of the segments n,n+ i,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 l)y 

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

 pression, of the phyletic stages in the ontogeny. Thus in the nauphus 

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

 subsequent thorax (Fig. iii^ IV-VIII), and in the Zo^a larva tlie 

 rudiments of six thoracic legs may be seen behind the ahcady 

 developed swimming-leg (Fig. 114, VI-XTII). 



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 IsojDod, which Hves 

 within other Crustaceans, a segment of the thorax is wanting in tlie 

 relatively well-developed larva, and in the Caprellida^ among the 

 Amphipod Crustaceans the whole abdomen of from six to seven st*g- 

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

 cases the gradual degeneration of the relative determinants has pre- 

 ceded step for step the degeneration of the part itself, and wlion this 

 is complete the ontogeny shows nothing of what was previously 

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



