348 Dr Barry on the Unity of Structure in 



merely an abstract notion of the mind, and exists nowhere in 

 Nature ; where there are only concrete realities, more or less 

 characteristic individualities, contained in a higher whole."" * 



Strictly speaking, therefore, no animal absolutely repeats in 

 its development, the structure of any part of any other animal ; 

 and not only is the human embryo at all periods of its existence 

 a human embryo, but the human heart and brain, closely as 

 they resemble corresponding organs in other Vertebrata at cer- 

 tain periods of development, are never any thing else than the 

 heart and brain of Man. If the young frog, the tadpole, resem- 

 bles in some respects a fish, and spends a portion of its existence 

 in the water, is it to be said that the tadpole is a fish ? Would 

 a highly developed fish constitute a frog ? Besides, as said by 

 Valentin,-)- a passage by the embryo of the so-called " higher" 

 animals through the " lower" grades, would imply the possibility 

 of an individual, at certain periods, laying down its individuality, 

 and assuming that of another animal ; which would abolish its 

 existence as a determinate concrete reality. 



No structure peculiarly characterizing- any one set of animals 

 in the perfect state, makes its appearance even in the embryonal 

 life of any other.J Thus the perfect gills of fishes, anJ the air- 

 sacs distributed through the body and the bones of birds, rela- 

 ting as these organs do, to the elements respectively, in which 

 fishes and birds have their abode, are found in them alone, 

 which could not be the case, did the so-called " higher" animals 

 pass through the perfect states of those said to be " lower ." 



Besides which, as Von Bar has truly said, were it a law of 

 nature, that individual development should consist in passing 

 through permanent but less elaborate forms, there is not a fea- 

 ture in embryonal life, nor a part then present, that we should 

 not expect to find, somewhere at least, in the animal kingdom. 

 Yet in what direction are we to look for an animal carrying 

 about its food, as the embryo the yolk, or a pendant portion of 



Valentin, Fragmente zu einer Kunftigen Gesetzlehre der individuellen 

 Entwickelung, in his Entwickelungsgeschichte des Menschen, &c. S. 591. 



t 1. c. p. 592. 



$ Valentin, 1. c. p. 596. 



Ueber Entwickelungsgeschichte derThiere, Beobachtung und Reflexion. 

 Konigsberg, 1828. S.204. 



