ioo PERIOD IV. 



insensible steps by which his own Palaeothere can be 

 seen to pass into the modern horse. Then we can 

 imagine how our regenerate Cuvier would draw nearer 

 and nearer to the common ancestor of the whole group, 

 a five-toed, plantigrade ungulate, with the full dentition 

 of forty-four unspecialised teeth, and how readily he 

 would admit that Phenacodus, both in its structure and 

 its geological horizon, was just the common ancestor 

 that theory required. The proofs of intermediate stages 

 between ancient and modern ungulates which he had 

 once called for in vain, he would now find ready to his 

 hand. It might well seem that the history of the 

 ungulates, with all its modern expansions, would suffice 

 to occupy even his unparalleled energy. He would see 

 with delight how the palaeontology which he had been 

 the first to treat as a science has enlarged the compara- 

 tive anatomy of which also he was so great a master. 

 He would cheerfully admit that both yield proofs of 

 that doctrine of descent with modification which a 

 hundred years ago seemed to him so questionable. 



Chamisso on the Alternation of Generations in Salpa. 



Trembley (see p. 57) had shown that Hydra, though 

 an animal, multiplies by budding like a plant. He got 

 indications, upon which he did not altogether rely, that 

 it also propagated by eggs, and ten years later (1754) 

 this supposition was confirmed by Roesel, who figured 

 the egg, though he was unable to demonstrate that a 

 young Hydra issues from it ; subsequent inquiry has 

 placed the fact beyond doubt. In 1819 Chamisso 

 announced that Salpa, a well-known Tunicate which 

 abounds at the surface of the sea, exhibits a regular 

 alternation of the two modes of increase, the egg-pro- 

 ducing form being succeeded by a budding form, the 



