26 



case, and all that gave form and character to the precedent 

 individual^ perish and are cast off: they are not changed 

 into the corresponding parts of the new individual. These 

 are due to a new and distinct developmental process ; ren- 

 dered possible through the retention of a certain proportion 

 of the unchanged germ-cells and nuclei. The process is 

 essentially the same as that which developes the cercariform 

 larva of the Distoma within the gregariniform one, or the 

 external bud from the Hydra, or the internal bud from the 

 Aphis. It is a slightly modified parthenogenesis : and the 

 phases by which the locomotive annelidous larva of the 

 Lernaea passes through the entomostracous stage before 

 retrograding to the final condition of the oviparous, limb- 

 less, bloated and rooted parasite, are much more those of 

 a metagenesis than a metamorphosis. 



The instances which I have adduced are a few out of 

 many that have been truly observed and faithfully re- 

 corded by zealous and acute men who have devoted them- 

 selves to the observant service of Nature, in hope of re- 

 ward by an insight into her secret operations for the multi- 

 plication of the species of the lower invertebrated classes 

 of animals. These observations have already demonstrated 

 the generation of many individuals from a single impreg- 

 nated ovum to be, in those classes, the rule and not the 

 exception i they have shown that the mode of that manifold 

 generation is similar in its essentials, though diverse in its 

 accessory circumstances, as in all other acts and things in 

 Nature. Bonnet, Chamisso, Von Baer, Ehrenberg, Dalyell, 

 Milne-Edwards, Sars, Loven, Siebold, Lister, Steenstrup, 

 Miiller, Kolliker and others have enriched their science 

 with valuable stores of such facts ; and they have mutually 

 corrected or confirmed the impressions derived from the 

 observation of the remarkable phenomena. These pheno- 

 mena remain, however, at present on record as such simply : 

 they have been generalized as being dependent on a law* : 

 * By Steenstrup, in the Essay above cited. 



