180 THE ORIGIN OF SPECIES 



Structure approaches the invertebrate classes. But mam- 

 mals 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. Phys- 

 iologists believe that the brain must be bathed by warm 

 blood to be highly active, and this requires aerial respi- 

 ration; 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, mem- 

 bers 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, marsu- 

 pials, edentata, and rodents, coexist in South America 

 in the same region with numerous monkeys, and prob- 

 -, ably interfere little with each other. Although organi- 

 zation, on the whole, may have advanced and be still 

 advancing throughout the world, yet the scale will al- 

 ways present many degrees of perfection; for the high 

 advancement of certain whole classes, or of certain mem- 

 bers of each class, does not at all necessarily lead to the 

 I extinction of those groups with which they do not enter 

 \ into close competition. In some cases, as we shall here- 

 \ after see, lowly organized forms appear to have been pre- 

 '' served to the present day, from inhabiting confined or 

 i peculiar stations, where they have been subjected to less 

 severe competition, and where their scanty numbers have 

 ^retarded the chance of favorable variations arising. 



Finally, I believe that many lowly organized forms 

 now exist throughout the world, from various causes. 



