Chap. IV.] ORGANISATION TENDS TO ADVANCE. 153 



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 conven- 

 iently 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 develop- 

 ment — 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 ani- 

 malcule — ^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, unim- 

 proved 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 



