NOTES TO BOOK XVIII. 091 



deposited are termed by Mr. Lyell metamorphic. 

 great extent of metamorphic rock changed by heat is now 

 uncontested. The internal changes which are produced by 

 the crystalline forces of mountain masses have been the 

 subjects of important and comprehensive speculations by 

 Professor Sedgwick. 



(IA.) p. 640. The hypothesis of the progressive deve- 

 lopement of species has been urged recently, in connexion 

 with the physiological tenet of Tiedemann and De Serres, 

 noticed Hist. Ind. Sci. B. xvn. c. vii. sect. 3 ; namely, 

 that the embryo of the higher forms of animals passes by 

 gradations through those forms which are permanent in 

 inferior animals. Assuming this tenet as exact, it has 

 been maintained that the higher animals which are found 

 in the more recent strata may have been produced by an 

 ulterior developement of the lower forms in the embryo 

 state ; the circumstances being such as to favour such a 

 developement. But all the best physiologists agree in 

 declaring that such an extraordinary developement of the 

 embryo is inconsistent with physiological possibility. Even 

 if the progression of the embryo in time have a general 

 correspondence with the order of animal forms as more 

 or less perfectly organized, (which is true in an extremely 

 incomplete and inexact degree,) this correspondence must 

 be considered, not as any indication of causality, but as 

 one of those marks of universal analogy and symmetry 

 which are stamped upon every part of the creation. 



Mr. Lyell (Principles, B. in. c. iv.) notices this doc- 

 trine of Tiedemann and De Serres ; and oberves, that 

 though nature presents us with cases of animal forms de- 

 graded by incomplete developement, she offers none of 

 forms exalted by extraordinary developement. Mr. LyelFs 



