362 PROVISIONAL HYPOTHESIS Chap. XXVII 



tis the normal stages of development, which are concealed 

 and hurried through or suppressed in most other insects. 

 In ordinary metamorphoses, the parts and organs appear to 

 become changed into the corresponding parts in the next 

 stage of development ; but there is another form of develop- 

 ment, which has been called by Professor Owen metagenesis. 

 In this case " the new parts are not moulded upon the inner 

 *' surface of the old ones. The plastic force has changed its 

 " course of operation. The outer 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," &c.^^ Metamorphosis, however, 

 graduates so insensibly into metagenesis, that the two pro- 

 cesses cannot be distinctly separated. For instance, in the 

 last change which Cirripedes undergo, the alimentary canal 

 and some other organs are moulded on pre-existing parts ; 

 but the eyes of the old and the young animal are developed 

 in entirely diiferent parts of the bod}^ ; the tips of the mature 

 limbs are formed within the larval limbs, and may be said 

 to be metamorphosed from them ; but their basal portions 

 and the whole thorax are developed in a plane at right angles 

 to the larval limbs and thorax ; and this may be called 

 metagenesis. Ihe metagenetic process is carried to an ex- 

 treme point in the development of some Echinoderms, for 

 the animal in the second stage of development is formed 

 almost like a bud within the animal of the first stage, 

 the latter being then cast off like an old vestment, yet 

 sometimes maintaining for a short period an independent 

 vitality.^" 



If, instead of a single individual, several were to be thus 

 developed metagenetically within a pre-existing form, the 

 process would be called one of alternate generation. The 

 young thus developed may either closely resemble the encasing 



** 'Parthenogenesis,' 1849, pp. 25, graduates into gemmation or zoid- 



26. Prof. Huxley has some excellent formation, which is in fact the same 



remarks (' Medical Times,' 1856, p. as metagenesis. 



637) on this subject in reference to ^^ Prof. J. Reay Greene, in Giin- 



the development of star -fishes, and ther's ' Record of Zoolog. Lit.,' 1865, 



shows how curiously metamorphosis p. 625. 



1 



