NATURAL SELECTION 101 



progressive development — it only takes advantage of such varia- 

 tions as arise and are beneficial to each creature under its complex 

 relations of life. And it may be asked what advantage, as far as 

 we can see, would it be to an infusorian animalcule — to an intes- 

 tinal worm — or even to an earth-worm, to be highly organized. If 

 it were no advantage, these forms would be left, by natural selec- 

 tion, unimproved or but little improved, and might remain for 

 indefinite ages in their present lowly condition. And geology tells 

 us that some of the lowest forms, as the infusoria and rhizopods, 

 have remained for an enormous period in nearly their present state. 

 But to suppose that most of the many now existing low forms have 

 not in the least advanced since the first dawn of life would be ex- 

 tremely rash; for every naturalist who has dissected some of the 

 beings now ranked as very low in the scale, must have been struck 

 with their really wondrous and beautiful organization. 



Nearly the same remarks are applicable, if we look to the dif- 

 ferent grades of organization within the same great group; for 

 instance, in the vertebrata, to the co-existence of mammals and 

 fish — among mammalia, to the co-existence of man and the orni- 

 thorhynchus — among fishes, to the co-existence of the shark and 

 the lancelet (Amphioxus), which latter fish in the extreme sim- 

 plicity of its structure approaches the invertebrate classes. But 

 mammals and fish hardly come into competition with each other; 

 the advancement 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 inhabit- 

 ing the water lie under a disadvantage in having to come contin- 

 ually to the surface to breath. With fishes, members of the shark 

 family would not tend to supplant the lancelet; for the lancelet, as 

 I hear from Fritz M filler, 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 advance- 

 ment 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 



