C;i.\r. IV. ORGANIZATION TENDS TO ADVANCE. 123 



to rise iu llie scale, liow is it that throughout tlie Avorkl a mul- 

 titude of the lowest forms still exist ; and how is it that in 

 each great class some forms are far more hiofhly developed than 

 otliers ? Why have not the more highly-developed forms 

 everywhere supplanted and exterminated the lower? Lamarck, 

 who lielieved iu an innate and ineWtable tendency toward per- 

 fection in all organic beings, seems to have felt this difficulty 

 so strongly, that he was led to suppose that new and simple 

 forms are continually being produced by spontaneous genera- 

 tion. Science, however, under her present aspect does not 

 countenance tlie belief, whatever the futiu-e may reveal, that 

 living creatures are now being generated. On our theory the 

 continued existence of lowly organisms oflcrs no difficulty ; for 

 natural selection, or the survival of the fittest, does not neces- 

 sarily include progressive development — it only takes advan- 

 tage of "such variations as arise and are bcnetlcial to each crea- 

 ture imder its complex relations of life. And it may be asked 

 what advantage, as far as we can see, would it be to an infu- 

 sorian animalcule — to an intestinal worm — or even to an earth- 

 worm, to be highly organized? If it were no advantage, tlu^se 

 forms would be left by natural selection miimproved or but 

 little improved; and might remain for indefmite ages in their 

 present little advanced condition. And geology tells us that 

 some of the lowest forms, as the infusoria and rizopods, 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 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 organ- 

 zation. 



Nearly the same remarks are applicable if we look to the 

 dilVt -rent grades of organization within each great group; for 

 instance, in the vertebrata, to the coexistence of manunals and 

 fish — among manunalia, to the coexistence of man and tlu; 

 ornitliorliynchus — among fishes, to the coexistence of the shark 

 and lancelet (Brancliiostoma), which latter fish in the extreme 

 8imi)licily of its structiue approaches the invertebrate classes. 

 But mammals and fisli hardly come into competition with each 

 other ; tiie advancement of tlie whole class of mammals, or of 

 certain memb(^rs in this class, to the highest grade would not 

 lead to their taking the place of, and thus exterminating, fishes. 

 Physiologists believe that the brain must be bathed by warm 



