164 ON THE DEGREE TO WHICH [Chap. IV. 



period in nearly their present state. But to suppose 

 that most of the many now existing low forms have not 

 in the least advanced since the first dawn of life would 

 be extremely rash; for every naturalist who has dis- 

 sected some of the beings now ranked as very low in the 

 scale, must have been struck with their really wondrous 

 and beautiful organisation. 



Nearly the same remarks are applicable if we look to 

 the difEerent grades of organisation within the same 

 great group; for instance, in the vertebrata, to the co- 

 existence of mammals and fish — amongst mammalia, to 

 the co-existence of man and the ornithorhynchus — 

 amongst fishes, to the co-existence of the shark and the 

 lancelet (Amphioxus), which latter fish in the extreme 

 simplicity of its structure approaches the invertebrate 

 classes. But mammals and fish hardly come into com- 

 petition with each other; the advancement of the whole 

 class of mammals, or of certain members in this class, to 

 the highest grade would not lead to their taking the 

 place of fishes. Physiologists believe that the brain 

 must be bathed by warm blood to be highly active, and 

 this requires aerial respiration; so that warm-blooded 

 mammals when inhabiting the water lie under a disad- 

 vantage in having to come continually to the surface to 

 breathe. With fishes, members of the shark family 

 would not tend to supplant the lancelet; for the lance- 

 let, as I hear from Fritz Miiller, has as sole companion 

 and competitor on the barren sandy shore of South 

 Brazil, an anomalous annelid. The three lowest orders 

 of mammals, namely, marsupials, edentata, and rodents, 

 co-exist in South America in the same region with nu- 

 merous monkeys, and probably interfere little with each 

 other. Although organisation, on the whole, may have 



