302 NATURAL HISTORY. 



organisation, they approach the ordinary Shell-fish, while 

 in others they show a relationship with the lower 

 Vertebrates. 



(3) It is a well-known embryological law that the 

 young animal in the early stages of its development 

 commonly possesses structures which it does not possess 

 in its adult state. It is also a well-known law that 

 structures which have only a temporary existence in the 

 embryo of one animal, are often found existing throughout 

 life in the adult of some other animal ; and that when 

 this occurs, the latter animal will occupy a lower position 

 in the animal scale than the former. Thus, the embryo of 

 the Quadrupeds possesses on each side of the neck a series 

 of transverse slits or fissures (the so-called 'visceral 

 clefts '), which lead down from the surface into the upper 

 part of the gullet (the ' pharynx'). In the adult Quad- 

 ruped no traces of these clefts are seen, only one of 

 them remaining at all (the opening of the ear), and that 

 only in a much modified form. On the other hand, the 

 embryo of the Fishes not only possesses these clefts, but 

 they are permanently retained, and are present therefore in 

 the adult, in which they become connected with the gills. 

 It seems, however, impossible to satisfactorily explain the 

 possession of visceral clefts by the mammalian embryo, 

 except upon the supposition that the Mammals and the 

 Fishes alike have descended from a common ancestor in 

 which these structures were present. The general fact, 

 therefore, that the embryos of animals so often possess 

 structures which are found in the adults of other animals, 

 is strongly in favour of the belief in the production of 

 animals by evolution from common ancestral types. 



