400 GENERAL PRINCIPLES 



For example, the wing of the bat is supported chiefly by the 

 enormously developed phalanges of the II, III, IV, and V 

 digits. In the embryo the bat hand is at first a normal pen- 

 tadactyl hand, and the great elongation of the four fingers 

 does not take place until very late. The bat being also a 

 Mammal follows the mammalian type of development, and 

 after it has reached the grade of a Mammal it continues its 

 development into the bat stage by transforming the mammalian 

 appendage into the more specialized bat appendage. 



812. If the above is a true conception of the origin of vesti- 

 geal and highly specialized organs then we are also in a position 

 to understand the parallelism of development described above. 

 It has been stated as a "Fundamental Law of Biogenesis" that 

 "the development of the individual recapitulates the history of 

 the race," which means that in its development each organism 

 passes through stages which represent the adult condition of 

 ancestral forms. 



813. The fish has four or five pairs of gill slits and between 

 the slits are the gill arches which bear the gills. Farther back 

 in the mid-ventral line lies the heart, from which a vessel runs 

 forward and divides into as many pairs of vessels as there are 

 pairs of gill arches. These vessels go one to each gill arch, and 

 above as many vessels pass from the arches to the mid-dorsal 

 line, where they unite into a single vessel, the dorsal aorta. 

 By these vessels the blood passes from the heart over the gills 

 for respiration. In the embryo of Amphibia, Reptiles, Birds 

 and Mammals, we also find these gill slits and the same arrange- 

 ment of blood vessels. In Amphibia this fish-like condition 

 persists in a functional manner until the time of metamorphosis 

 of the tadpole. But in the other three classes respiration by 

 gills never occurs and the gill slits are functionless rudiments. 

 For the arrangement of the several pairs of vessels which pass 

 over these functionless gill arches there is also no explanation 

 to be offered except that they have been inherited from fish-like 



