4 THE THEORY OF EVOLUTION 



sciences which deal with the living world and to-day 

 almost completely controls scientific thought. . . . 

 And yet science does not sincerely rejoice in its con- 

 quests. Only a few incorrigible and uncritically dis- 

 posed optimists steadfastly proclaim what glorious 

 progress we have made; otherwise, in scientific as 

 in lay circles, there prevails a widespread feeling of 

 uncertainty and doubt. Not as though the correctness 

 of the principle of descent were seriously questioned; 

 rather does the conviction steadily grow that it is in- 

 dispensable for the comprehension of living nature, 

 indeed self-evident. [Italics mine.] But never before 

 has it become so notorious as in the last decade, how 

 little there is in this theory that is universally ac- 

 cepted, as appears when the most obvious questions 

 are asked regarding the course of development and 

 its efficient causes. No one can supply simple and 

 satisfactory answers. Should any one ask, by what 

 steps the placental mammals have proceeded from 

 lower quadrupeds, he is informed that some inves- 

 tigators derive them from the reptiles by way of 

 the marsupials, others directly from certain Permian 

 reptiles, while others again push back their origin 

 to the most ancient periods and derive them from 

 entirely unknown ancestors, or, should somebody 

 wish to know why the many remarkable animals 

 and plants of preceding geological periods are no 

 longer living, and what causes led to their extinction, 

 he would receive anything but a clear and unam- 

 biguous answer. Who would venture to-day to 



