238 CLASSICS OF MODERN SCIENCE 

 and rhizopods, have remained for an enormous 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 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 lancelot 

 {Amphioxus) y 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 disadvantage 

 in having to come continually to the surface to breathe. With fishes, 

 members of the shark family would not tend to supplant the lance- 

 let ; for the lancelet, as I hear from Fritz Miiller, has as sole compan- 

 ion 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 numerous monkeys, and probably interfere little 

 with each other. Although organisation, on the whole, may have 

 advanced and be still advancing throughout the world, yet the scale 

 will always present many degrees of perfection ; for the high ad- 

 vancement of certain whole classes, or of certain members of each 

 class, does not at all necessarily lead to the extinction of those groups 

 with which they do not enter into close competition. In some cases, 

 as we shall hereafter see, lowly organised forms appear to have been 

 preserved to the present day, from inhabiting confined or peculiar 

 stations, where they have been subjected to less severe competition, 

 and where their scanty numbers have retarded the chance of favour- 

 able variations arising. 



Finally, I believe that many lowly organised forms now exist 



