88 ESSAY ON CLASSIFICATION 



the limits of the four great branches of the animal kingdom and that 

 general homology, strictly proved, proves also typical identity, as 

 special homology proves class identity. 



The results of all embryonic investigations of modern times go to 

 show more and more extensively that animals are entirely inde- 

 pendent 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 was supposed that the embryo could be affected 

 directly by external influences 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 influ- 

 ence 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 other^vise reach them in the various structures animals build 

 for the protection of their young or of their eggs, show distinctly 

 that the instinct of all animals leads them to remove their progeny 

 from the influence of physical agencies, or to make these agents sub- 



^■^ This seems the most appropriate place to remark that the distinction made be- 

 tween viviparous and oviparous animals is not only untenable as far as their first 

 origin in the egg is concerned, but also unphysiological, if it is intended by this 

 designation to convey the idea of any affinity or resemblance in their respective modes 

 of development. Fishes show more distinctly than any other class that animals, the 

 development of which is identical in all leading features, may either be viviparous 

 or oviparous; the difference here arising only from the connection in which the egg 

 is developed and not from the development itself. Again, viviparous and oviparous 

 animals of different classes differ gieatly in their development, even though they may 

 agree in laying eggs or bringing forth living yoimg. The essential feature upon which 

 any important generalization may be based is of course the mode of development of 

 the germ. In this respect we find that Selachians, whether oviparous or viviparous, agree 

 with one another; this is also the case with the bony fishes and the reptiles, whether 

 they are respectively oviparous or viviparous; even the placentalian and non-placen- 

 talian 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 the germ 

 is developed, to the exclusion of the leading features of the transformations of the 

 germ itself. 



