92 ON THE DEGREE TO WHICH [CHAP. IV. 



that all organic beings are striving to increase at a high ratio and 

 to seize on every unoccupied or less well occupied place in the 

 economy of nature, that it is quite possible for natural selection 

 gradually to fit a being to a situation in which several organs 

 would be superfluous or useless : in such cases there would be 

 retrogression in the scale of organisation. Whether organisation 

 on the whole has actually advanced from the remotest geological 

 periods to the present day will be more conveniently discussed in 

 our chapter on Geological 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 everywhere supplanted 

 and exterminated the lower ? Lamarck, who believed in an innate 

 and inevitable tendency towards 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 continually 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 selection, or the survival of the fittest, does not necessarily 

 include 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 infusorian animalcule to 

 an intestinal worm or even to an earth-worm, to be highly or- 

 ganised. If it were no advantage, these forms would be left, by 

 natural selection, 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 extremely rash; for every naturalist who has dis- 

 sected some of the beings now ranked as very low in the scale, 

 must have been struck with their really wondrous and beautiful 

 organisation. 



Nearly the same remarks are applicable if we look to the different 

 grades of organisation within the same great group ; for instance, 

 in the vertebrata, to the co-existence of mammals and fish amongst 

 mammalia, to the co-existence of man and the ornithorhynchus 

 amongst fishes, to the co-existence of the shark and the lancelet 

 (Amphioxus), which latter fish in the extreme simplicity of its struc- 

 ture approaches the invertebrate classes. But mammals and fish 

 hardly come into competition with each other ; the advancement of 



