OB&ANIZATION TENDS TO ADVANCE. 119 



approaches the invertebrate classes. But mammals and 

 fish hardly come into competition with each other; the ad- 

 vancement 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 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. Tlie 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 organization, 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 here- 

 after see, lowly organized 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 competion, and where their scanty numbers have re- 

 tarded the chance of favorable variations arising. 



Finally, I believe that many lowly organized forms now 

 exist throughout the world, from various causes. In some 

 cases variations or individual differences of a favorable 

 nature may never have arisen for natural selection to act 

 on and accumulate. In no case, probably, has time suf- 

 ficed for the utmost possible amount of development. In 

 some few cases there has been what _ we must call retro- 

 gression or organization. But the main cause lies in the 

 fact that under very simple conditions of life a high organi- 

 zation would be of no service, — ^possibly would be of actual 

 disservice, as being of a more delicate nature, and more 

 liable to be put out of order and injured. 



Looking to the first dawn of life, when all organic beings. 



