NATURAL SELECTION 99 



tinue to increase. But which groups will ultimately prevail, no man 

 can predict; for we know that many groups, formerly most exten- 

 sively developed, have now become extinct. Looking still more 

 remotely to the future, we may predict that, owing to the continued 

 and steady increase of the larger groups, a multitude of smaller 

 groups will become utterly extinct, and leave no modified descend- 

 ants; and consequently, that, of the species living at any one 

 period, extremely few will transmit descendants to a remote fu- 

 turity. I shall have to return to this subject in the chapter on 

 classification, but I may add that as, according to this view, ex- 

 tremely few of the more ancient species have transmitted descend- 

 ants to the present day, and as all the descendants of the same 

 species form a class, we can understand how it is that there exist 

 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 ORGANIZATION 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 

 improvement inevitably leads to the gradual advancement of the 

 organization 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 organization. Among the vertebrata the degree of intel- 

 lect 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 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 ani- 

 mal 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 specialization 

 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, 



