CHAP. IV.] ORGANISATION TENDS TO ADVANCE. 93 



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. 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 

 preserved to the present day, from inhabiting confined or peculiar 

 stations, where they have been subjected to less severe competi- 

 tion, and where their scanty numbers have retarded the chance of 

 favourable variations arising. 



Finally, I believe that many lowly organised forms now exist 

 throughout the world, from various causes. In some cases 

 variations 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 development. In some few cases there has been what 

 we must call retrogression of organisation. But the main cause 

 lies in the fact that under very simple conditions of life a high 

 organisation 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 steps 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, his law " that homologous 

 units of any order become differentiated in proportion as their 

 relations to incident forces become different " would come into 

 action. But as we have no facts to guide us, speculation on the 

 subject is almost useless. It is, however, an error to suppose that 



