118 ON THE DEOREE TO WHICH 



day will be more conveniently discussed in our chapter on 

 G-eological Succession. 



But it may be objected that if all organic beings thus 

 tend to rise in the scale, how is it that throughout the 

 world a multitude of the lowest forms still exist; and how 

 is it that in each great class some forms are far more 

 highly developed than others? Why have not the more 

 highly developed forms every where supplanted and exter- 

 minated the lower? Lamarck, who believed in an innate 

 and inevitable tendency toward perfection in all organic 

 beings, seems to have felt this difficulty so strongly that he 

 was led to suppose that new and simple forms are continu- 

 ally being produced by spontaneous generation. Science 

 has not as yet proved the truth of this belief, whatever the 

 future may reveal. On our theory the continued existence 

 of lowly organisms offers no difficulty; for natural selec- 

 tion, or the survival of the fittest, does not necessarily in- 

 clude progressive development — it only takes advantage of 

 such variations 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 in- 

 fusorian animalcule — to an intestinal worm — or even to au 

 earth-worm, to be highly organized. If it were no advan- 

 tage, these forms would be left, by natural selection, un- 

 improved or but little improved, and might remain for in- 

 definite ages in their present lowly condition. And geol- 

 ogy tells us that some o'f 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 ad- 

 vanced since the first dawn of life would be extremely 

 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 organiza- 

 tion. 



_ Nearly the same remarks are applicable, if we look to the 

 different grades of organization within the same great 

 groi|p; for instance, in the vertebrata, to the co-existence 

 of mammals and fish — among mammalia, to the co-exist- 

 ence of man and the ornithorhynchus — among fishes, to 

 the co-existence of the shark and thelancelet(Amphioxus), 

 which latter fish in the extreme simplicity of its structure 



