Chap. I. 



METAMOlirilOSES OF ANIMALS 



87 



of the animal kingdom, and that general homology strictly proved, proves also 

 typical identity, as special homology proves class identity. 



The results t)f all embryonic investigations of modern times go to show more 

 and more extensively', that animals are entirely independent of external causes in 

 their development. The identity of the metamorphoses of oviparous and viviparous 

 animals belonging to the same type, furnishes the most convincing evidence to that 

 effect.^ Formerly it Avas supposed that the embryo could be affected du'ectly by 

 external intluences to such an extent, that monstrosities, for instance, were ascribed to 

 the influence of external causes. Direct observation has shown, that they are 

 founded upon peculiarities of the normal course of their development.^ The snug 

 berth in which the young undergo their first transformation in the womb of their 

 mother in all Mammalia, excludes so completely the immediate influence of any 

 external agent, that it is only necessary to allude to it, to show how independent 

 their growth must be of the circumstances in which even the mother may be placed. 

 This is equally true of all other viviparous animals, as certain snakes, certain sharks, 

 and the viviparous fishes. Again, the uniformity of temperature in the nests of birds, 

 and the exclusion, to a certain degree, of influences which might otherwise reach 

 them, in the various structures animals build for the protection of their yoimg or of 

 their eggs,^ show distinctly, that the instinct of all animals leads them to remove 

 their progeny from the influence of jihysical agencies, or to make these agents sub- 

 sei'vient to their purposes, as in the case of the ostrich. Reptiles and terrestrial 

 Mollusks bury their eggs to subtract them from varying influences; fishes deposit 

 them in localities where they are exposed to the least changes. Lasects secure theirs 



' This seems the most appropriate plaee to re- 

 mark, that the distinction made lidween viviparous 

 and oviparous animals is not only untenable as far as 

 their first origin in the egg is concerned, but also un- 

 pli_vsi()l()<rical, if it is intended, by this desijrnation, to 

 convey tlie idea of any atflnity or resemblance in their 

 respective modes of development. Fishes show more 

 distinctly than any other class, that animals, the devel- 

 opment of whidi is identical, in all its leading feat- 

 ures, may either be vivijiarous or oviparous ; the dif- 

 ference here arising only from the connection in 

 which the egg is developed, and nut I'njiu ilic devel- 

 opment itself. Again, viviparous and oviparous ani- 

 mals of ditfcrent classes ditfer greatly in tiieir devel- 

 opment, even though they may agree in laying eggs 

 or bringing fortii living young. The essential feature 

 upon which any iniporlant generalization may be 



based, is, of course, the mode of development of the 

 germ. In this respect we find that Selachians, whe- 

 ther oviparous or viviparous, agree with one another ; 

 this is also the case with the bony fishes and the rep- 

 tiles, whether they arc respectively oviparous or vivi- 

 parous ; even the placentalian and non-placentalian 

 Mammalia agree with one another in what is essential 

 in their development. Too much importance has thus 

 far been attached to the connections in which liie germ 

 is developed, to the exclusion of the leading features 

 of the transformations of the germ itself. 



^ BisiiOFF, (Tii. L. W.,) in U. Wagner's Iland- 

 wiirterbuch der Physiologic, Article " Entwickelungs- 

 geschichte," p. 885. 



' Burdach's Physiologic, etc., q. a, vol. 2, 2d ed. 

 Sect. 334r-38. See, also, Kiiun- and Spence's Intro- 

 duction, etc., q. a. 



