EFFECTS OF NATURAL SELECTION 137 



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 be- 

 ings 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 mam- 

 mals 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 in- 

 vertebrate 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 con- 

 tinually 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 com- 

 panion 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 or- 

 ganisation, 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 advance- 

 ment 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 competi- 

 tion. In some cases, as we shall hereafter see, lowly or- 

 ganised 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 fav- 

 orable variations arising. 



