GENERAL LAWS OF ANIMAL STRUCTURE. 243 



but cellular, like the lung of a frog. It is really a lung, 

 and these animals breathe partly by gills as a fish, and 

 partly by lungs as an amphibian reptile. It is an am- 

 phibian fish. 



(b) Now, the condition last described is exactly that 

 of the lowest amphibians and the early or larval condi- 

 tion of all amphibians. In fact, in the development of 

 the individual, in 1 the individual life history of an am- 

 phibian, we have all the stages of change of the respira- 

 tory organs from the complete gill-breathing stage to the 

 complete lung-breathing stage. The very young tad- 

 pole breathes wholly by gills, like a fish ; the frog wholly 

 by lungs, like a reptile or a mammal. How did the 

 change take place ? By modification of the gills ? No 

 — -but by the development of another organ in the position of 

 the air bladder of a fish. At first, as already said, the 

 breathing is wholly by gills, like a fish. Then by the de- 

 velopment of an organ in the position of the air bladder 

 of the fish and connected with the throat it begins to 

 breathe also by a commencing lung. It now breathes 

 equally by gills and by lungs, both water and air. This 

 is the permanent condition of the highest fishes (lung 

 fishes) and of the lowest amphibians (the Perenni- 

 branchiata), such as the siren, etc. From this time onward 

 the gills decrease and the lungs increase, until in the 

 adult the gills disappear and the breathing is wholly by 

 lungs. The first argument (a) is strongly presumptive; 

 the second (b) is demonstrative. 



It is unnecessary to give further examples from ani- 

 mals, as all that we shall have to say in this chapter will 

 be' a continuous illustration of homology; but a few 

 illustrations from plants will show the universality of the 

 principle. 



(1) A rhizome or rootstock — such as that of a flag, 

 or of calamus, or of a fern — is analogous to a root, for 



