FOUND LINKS. 137 



in the early life of all quadrupeds, without exception, 

 there are to be perceived evidences of their connexion 

 with lower forms of life. Thus, every Vertebrate, at 

 an early stage of its development, exhibits certain 

 clefts or openings in the sides of the neck, known as 

 branchial clefts, and which are bounded by folds 

 called branchial arches. These, in fishes, come to 

 bear the gills, but in reptiles, birds, and quadrupeds, 

 they simply disappear useless rudiments of struc- 

 tures, once necessary in the life of aquatic quadruped- 

 ancestors, and still retained in the developments of 

 to-day by the law of inheritance. Thus, in the deve- 

 lopment of a rabbit, the biologist sees three pairs of 

 branchial arches behind the mouth of the embryo, 

 and four branchial clefts. Three of the clefts dis- 

 appear, and the fourth, by the modification to which 

 development has been subjected in the evolution of 

 the quadruped tribes, is converted into the Eustachian 

 tube and other structures belonging to the ear. The 

 presence of (< branchial clefts" in the developing 

 mammal would alone suffice to show its evolution 

 from lower life. Denying that probability, which to 

 the biologist is a fact, there is no explanation what- 

 ever of the cause or existence of these vanishing struc- 

 tures in the history of the quadruped race. 



Concentrating our attention on the Monotremes 

 themselves, however, we may speedily discover nume- 

 rous links which unite them with lower life, and 

 specially with the bird-type. There, firstly, exists in 

 these quadrupeds what Huxley has called " a striking 



