154 ON THE DEOKEE TO WHICH [Chap. TV. 



period in nearly their present state. But to suppose that 

 most of the niany 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 dissected 

 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 different 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 competition 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 lancelet, 

 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 mam- 

 mals, namely, marsupials, edentata, and rodents, co-exist 

 in South America in the same region with numerous 

 monkeys, and probably interfere little with each other. 

 Although organisation, on the whole, may have advanced 



