Chap. IV.] NATURAL SELECTION. 97 



more ancient species have transmitted descendants to the present 

 day, and, as all the descendants of the same species form a class, we 

 can understand how it is that there exists so few classes in each 

 main division of the animal and vegetable kingdoms. Although 

 few of the most ancient species have left modified descendants, yet, 

 at remote geological periods, the earth may have been almost as 

 well peopled with species of many genera, families, orders, and 

 classes, as at the present time: 



On the Degree to which Organisation tends to advance. 



Natural Selection acts exclusively by the preservation and accu- 

 mulation of variations, which are beneficial under the organic and 

 inorganic conditions to which each creature is exposed at all periods 

 of life. The ultimate result is that each creature tends to become 

 more and more improved in relation to its conditions. This im- 

 provement inevitably leads to the gradual advancement of the 

 organisation of the greater number 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 organisation. 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 specialisa- 

 tion for different 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 instance, to fishes, 

 amongst which some naturalists rank those as highest which, like 

 the- sharks, approach nearest to amphibians ; whilst 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, amongst which the standard of intel- 

 lect 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, 



