116 ON THE DEGREE TO WEIOH 



tiniie to increase. But which groups will ultimately pre- 

 vail, uo man can predict; for we know that many groups, 

 formerly most extensively developed, have now become ex- 

 tinct. 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 be- 

 come utterly extinct, and leave no modified descendants; 

 and consequently that, of the species living at any one 

 period, extremely few will transmit descendants to a 

 remote futurity. I shall have to return to this subject in the 

 chapter on classification, but I may add that as, according 

 to this view, extremely few of the 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 exist so few classes in 

 each main division of the animal and vegetable kingdoms. 

 Although few of the most ancient species have left modi- 

 fied 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. 



GIT THE DEGEEE TO WHICH ORSAJTIZATION 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 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 natural- 

 ists have not defined to each other's satisfaction what is 

 meant by an advance in organization. Among the verte- 

 brata the degree of intellect and an approach in structure 

 to man clearly come into play. It might be j;hought 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 



