Page - 135 - in The Origin of Species

EFFECTS OF NATURAL SELECTION 13S creature tends to become more and more improved in relation to its conditions. This improvement inevitably leads to the gradual advancement of the organisation of the greater num- ber of living beings throughout the world. But here we enter on a very intricate subject, for naturalists have not defined to each other's satisfaction what is meant by an advance in organization. Amongst 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 development from the 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 specialisation for dif- ferent functions ; or, as Milne Edwards would express it, the completeness of the division of physiological labour. But we shall see how obscure this subject is if we look, for in- stance, to fishes, amongst which some naturalists rank those as highest which, like the sharks, approach nearest to amphi- bians; whilst other naturalists range 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, amongst 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, 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 organisation, the amount of differentiation and specialisation of the several organs in each being when adult (and this will include the advance- ment of the brain for intellectual purposes), natural selec- tion clearly leads towards this standard: for all physiologists admit that the specialisation of organs, inasmuch as in this