Chap. IY.J NATUKAL SELECTION. 151 



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 ivhich Organisation tends to advance. 



Natural Selection acts exclusively by the preservation 

 and accumulation of variations, which are beneficial 

 under the organic and inorganic conditions to which 

 each creature is exposed at all periods of life. The ulti- 

 mate result is that each creature tends to become more 

 and more improved in relation to its conditions. This 

 improvement inevitably leads to the gradual advance- 

 ment 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 in- 

 tellect 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 



