Chap. IV.] ORGANISATION TENDS TO ADVANCE. 153 



several organs would be superfluous or useless : in such 

 cases there would be retrogression in the scale of organi- 

 sation. "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 ex- 

 terminated 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 



