Chap. iV.J ORGANISATION TENDS TO ADVANCE. 99 



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 bo 

 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 oraithorhynchus — 

 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 advance- 

 ment 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 sur- 

 face 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 mammals, namely, marsupials, edentata, and 

 rodents, co-exist in South America in the same region with nume- 

 rous 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 advancement 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 pre- 

 served 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 

 throughout the world, from various causes. In some cases varia- 

 tions or individual differences of a favourable nature may never 

 have arisen for natural selection to act on and accumulate. In no 

 case, probably, has time sufficed for the utmost possible amount of 



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