112 ON the'degree to which 



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 hav- 

 ing to come continually to the surface to breathe. With 

 fishes, members of the shark family would not tend to sup- 

 plant 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, eden- 

 tata, 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 cer- 

 tain 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 organized 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 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 sufficed 

 for the utmost possible amount of development. In some 

 few cases there has been what we must call retrogression of 

 organization. But the main cause lies in the fact that under 

 very simple conditions of life a high organization 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, 

 as we may believe, presented the simplest structure, how, 

 it has been asked, could the first step in the advancement or 

 differentiation of parts have arisen ? Mr. Herbert Spencer 

 would probably answer, that, as soon as simple unicellular 

 organism came by growth or division to be compounded of 

 several cells, or became attached to any supporting surface^ 



