THE BIOOENKTIC LAW 187 



such appxivntly simple tvansforniations, tl\o oldor stage does not, in 

 every ontogeny, preeede tl\e moiv veeent one as a preparation for it, 

 thoxigh 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 

 occurivnce of sonietliing really tpiite new. Tlierel'ore snch a uew 

 st^\g^? cm\ arise only from the germ-plasm, by the duplication, and in 

 jwrt variation, of the determinants of tlie precetiing stage. If. for 

 instance, the body of a Cr\istaeean be lengthened by a segment, this 

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

 intelligible enough that the uew segment can be formed iu the 

 ontogeny only after the development of the older pi-eceding one. for 

 its determinants eome fixtm that, and are fi-om the beginning so 

 ju-raiiged tliat they ai-e only liberated to activity by the formation 

 of the preceding segnneut. 



Now, if ill the eoxii-se of the phylogeny numeix>us new segments 

 were added to the body of the Crnstaeeau. the ontogeny would be 

 materially prolonged, and condenstition would become neeessiiry 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 inciv^ising rapidity 

 in the ontogeny, until tinally they appeared i^imultinie^nish/: the 

 determinants of the segments ii,n + i, n + z. . . . n+.v varied iu regai-d 

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

 successively, but simultaneously, in the cell complexes controlled by 

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

 pi"essiou, of the phyletie stages in the ontogeny. Thus in the uauplius 

 of Leptodora we see the rudiments of tive of the paire of legs of the 

 sul^sequeut thoi-ax (Fig. iii, IV~VIII), and in the Zowa larva the 

 rudiments of six thoracic leg's may be seen behind the ;dready 

 developed swimming-leg (V^ig. 114, VI-XTII). 



But in tlie couiise 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 

 Avithin other Crustaceans, a segment of the thorax is wanting in the 

 relatively well-developed hu-va, aud in the Caprellida; timong the 

 Amphipod l.>ustaceans the whole abdomen of fi-om six to seven seg- 

 ments hjii! degenerated to a naiTow, rudimentary structure. In sucli 

 cases tlie gradual degeneration of the relative determinants h!\s pre- 

 ceded st4?p for step the degeneration of the part itself, and when tliis 

 is complete the ontogeny shows nothing of what Wiis previously 

 pi-esent and so we may speak of a ' falsilication " of the phylogeny. 



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