['^2 <JN THE DEGKEE TO WHICH Cuap. IV. 



incnt from the cnil)ryo to maturity would suffice as a standard 

 of comparison ; but there are cases, as with certain parasitic 

 crustaceans, in Avhich several parts of the structure become 

 less perfect, so that the mature animal cannot be called higher 

 than its larva. Von Baers standard seems the most widely 

 applica])le and the Ix'st, namely, the amount of diflerentiation 

 of the diiferent parts of the same organic being, in the adult 

 state as I should be inclined to add, and their specialization 

 for difl'ercnt functions ; or, -as Milne Edwards would express it, 

 the completeness of the division of physiological labor. But we 

 shall sec how obscure this subject is if Ave look, for instance, to 

 fish, among which some naturalists rank those as highest which, 

 like the sliarks, approach nearest to amphibians ; while other 

 naturalists rank the common bony or teleostean fishes as the 

 highest, inasmuch as they are most strictly fish-like, and differ 

 most from the other vertebrate classes. Still more plainly we 

 see the obscurity of the subject by turning to plants, among 

 which the standard of intellect is of course quite excluded ; and 

 here some botanists rank those plants as highest which have 

 every organ, as sepals, jietals, stamens, and pistils, fully devel- 

 oped in each flower ; whereas other botanists, 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 organization, the 

 amount of differentiation and speciahzation of the several or- 

 gans in each being when adult (and this will include the ad- 

 vancement of the brain for intellectual purposes), natural selec- 

 tion clearly leads toward highness ; for all physiologists admit 

 that the specialization of organs, inasmuch as they perform in 

 this state their functions better, is an advantage to each being; 

 and hence the accumulation of variations tending toward spe- 

 cialization is within the scope of natural selection. On the 

 other hand, Ave can see, bearing iu mind that all organic beings 

 are striving to increase at a high ratio and to seize on every 

 ill-occupied place in the economy of Nature, that it is quite pos- 

 sible for natural selection gradually to fit an organic being to 

 a situation in AA'hich scA^eral organs would be superfluous or 

 useless : in such cases there might be retrogression in the scale 

 of organization, AN'hcther organization on the Avhole has 

 actually adA'anced from the remotest geological jieriods to the 

 present day Avill be more conveniently discussed in om- chapter 

 on Geological Succession. 



But it may be objected that, if all organic beings thus tend 



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