OBJECTIONS TO THE THEORY OF DESCENT. 315 



orders of mammals, 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 adyanced 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 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 favor- 

 able 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 develop- 

 ment. 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 struct- 

 ure, how, it has been asked, could the first steps in the 

 advancement or differentiation of parts have arisen ? 



