12i ADVANCE OF ORGANIZATION. Chap. IV. 



blood to be highly active, and tliis I'cquires auiial respiration ; 

 so that Avarm-bloodod maininals when inhabiting the water live 

 under some disadvantages in comparison with iishes. In this 

 latter class, 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, coexist in South America in the same region with 

 numerous monkeys, and probably interfere little with each 

 other. Altliougli oi'ganization, on the whole, may have ad- 

 vanced 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 mem- 

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

 tinction of those groups with Avhich 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 confmed or peculiar stations, where 

 they have been subjected to less severe competition, and where 

 their scant}- numbers have retarded the chance of favorable 

 variations arising. 



Finally, I believe that many lowly-organized forms now 

 exist throughout the Avorld, 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 Avhat we must call retrogression of organization. 

 But the main cause lies in the fact that under very simple con- 

 ditions 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 w^e may believe, presented the simplest structun^, how, it 

 has been asked, couhl the first steps in the advancement or 

 differentiation of parts have arisen'? Mr. Herbert S])encer 

 would probably answer that as soon as simple unicellular or- 

 ganism came by growth or division to be coinj^ounded of sev- 

 eral cells, or became attached to any supporting surface, his 

 law " that homologous imits of any order become ilifferentiated 

 in proportion as their relations to incident forces become differ- 



