PREFACE xi 



Let us suppose a group or section of the old form 

 severed from the main body by changes of the earth's 

 surface and so removed to a new habitat, it would at 

 once be encompassed and laid hold of by the modifying 

 influences of its changed material surroundings, and 

 would in this manner be adapted to its new environ- 

 ment. When we think of the innumerable changes 

 that the earth's surface has undergone, owing to 

 depressions and elevations, now turning seabeds into 

 dry land, now sinking dry land so as to form seabeds, 

 it will at once seem probable that all the various 

 species of a family derived from an old and now 

 extinct genus owe their existing specific differentiations 

 to having been frequently compelled to migrate, and 

 frequently brought under new external conditions. 



According to this principle of evolution (the only 

 one, I repeat, of which Science has any certain 

 knowledge), organic beings are mere passive recipients 

 of the modifying forces of Nature, without possessing 

 in themselves the slightest power of choosing on their 

 own account what is good or beneficial for their 

 particular organic development. 



But while, from the results of observation and 

 experience, we can thus reasonably infer that the 

 evolutionary power of Nature has acted from the 

 genus that founded the family group modifying the 

 species derived from it, Nature's evolutionary agency, 

 so far as we can trace its effects, appears unable to 

 pass beyond the bounds of a family so as to originate 

 a new generic type. 



