136 ORIGIN OF SPECIES 



state they perform their functions better, is an advantage to 

 each being; and hence the accumulation of variations tending 

 towards specialisation is within the scope of natural selection. 

 On the other hand, we can see, bearing in mind that all or- 

 ganic 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 selec- 

 tion 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 Succes- 

 sion. 



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? La- 

 marck, 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 spon- 

 taneous 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 organised. 

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

 selection, unimproved or but little improved, and might re- 

 main 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 



