30 THE MAMMALIA. 



And hence every class of the Mammalia of the 

 present day can be understood only from its con- 

 nection with its geological ancestors. But are we, 

 by admitting true agreement in organisms and their 

 parts, and by only allowing what is inherited to be 

 real homologies, are we entering a domain wholly 

 distinct from that of convergence? On the con- 

 trary. Inheritance is only a case of repetition 

 under the same conditions, a case of universal 

 law. The whole Darwinian principle of selection 

 and progress finds its application also in the 

 generalisation of the doctrine of repetitions. 



THE DISTINCTIVE CHARACTERISTICS OF THE 

 MAMMALIA. 



The earlier zoologists, with Linnaeus, classed 

 together as one species all those individuals * that 

 agree in essential points and have been descended 

 unchanged from the same ancestors.' The later 

 zoologists replace this supposed straight line of 

 descent, which involves the miracle of creation 

 and also the future invariability of organisms, by 

 including the agency of variability dependent upon 

 time and circumstances. As long as animal beings 

 of a supposed or proved common descent agree in 

 form and structure, we leave them together as one 



