og On the Degree to which Chap. iv. 



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probably with more truth, look at the plants which have their 

 several organs much modified and reduced in number as the 



highest. 



If we take as the standard of high organisation, the amount of 

 differentiation and specialisation of the several organs in each being 

 when adult (and this will include the advancement of the brain for 

 intellectual purposes), natural selection clearly leads towards this 

 standard : for all physiologists admit that the specialisation of organs, 

 inasmuch as in this 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 selec- 

 tion. On the other hand, we can see, bearing in mind 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 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 

 yariations as arise and are beneficial to each creature under its com- 

 plex 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 in- 

 testinal worm — or even to an earth-worm, to be highly organised. 

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

 tion, 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* 



