NATURAL SELECTION 177 



ization of the greater number of living beings throughout ' 

 the world. But here we enter on a very intricate sub- 

 ject, for naturalists have not defined to each other's sat- 

 isfaction what is meant by an advance in organization. 

 Among the vertebrata the degree of intellect and an 

 approach in structure to man clearly come into play. 

 It might be thought that the amount of change which 

 the various parts and organs pass through in their devel- 

 opment from the embryo to maturity would suffice as a 

 standard of comparison; but there are cases, as with cer- 

 tain 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 stan- 

 dard 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 in- 

 clined to add, and their specialization for different func- 

 tions; or, as Milne Edwards would express it, the com- 

 pleteness of the division of physiological labor. But we 

 I 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 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. We see still more plainly the 

 obscurity of the subject by turning to plants, among 

 which the standard of intellect is of course quite ex- 

 cluded; and here some botanists rank those plants as 

 highest which have every organ, as sepals, petals, sta- 

 mens, and pistils, fully developed in each flower; whereas 

 other botanists, probably with more truth, look at the 



