110 ON THE DEGREE TO WHICH 



into play. It might be thought that the amount of change 

 which the various parts and organs pass through in their 

 development from embryo to maturity would suffice as a 

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

 parasitic crustaceans, in which several parts of the structure 

 become less perfect, so that the mature animal cannot be 

 called higher than its larva. Von Baer's standard seems the 

 most widely applicable and the best, namely, the amount of 

 differentiation of the parts of the same organic being, in the 

 adult state, as I should be inclined to add, and their special- 

 ization for different functions ; or, as Milne Edwards would 

 express it, the completeness of the division of physiological 

 labor. But we shall see how obscure this subject is if we 

 look, for instance, to fishes, among which some naturalists 

 rank those as highest which, like the sharks, approach near- 

 est 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. We see still more plainly the obscurity 

 of the subject by turning to plants, among which the stand- 

 ard of intellect is of course quite excluded; and here some 

 botanists rank those plants as highest which have every 

 organ, as sepals, petals, stamens, and pistils, fully developed 

 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 specialization of the several organs in 

 each being when adult (and this will include the advance- 

 ment of the brain for intellectual purposes), natural selection 

 clearly leads toward this standard: for all physiologists 

 admit that the specialization of organs, inasmuch as in this 

 state they perform their functions better, is an advantage to 

 each being ; and hence the accumulation of variations tend- 

 ing toward specialization is within the scope of natural 

 selection. 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 econorny 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 

 organization. Whether organization on the whole has actu- 

 ally advanced from the remotest geological periods to the 



