PREFACE. v 
distinct and persistent for each group as are 
the fossil types of each period revealed to us by 
the latter ; and that the experiments upon domes- 
ticated animals and cultivated plants, on which 
its adherents base their views, are entirely foreign 
to the matter in hand, since the varieties thus 
brought about by the fostering care of man are 
of an entirely different character from those ob- 
served among wild species. And while their 
positive evidence is inapplicable, their negative 
evidence is equally unsatisfactory, since, however 
long and frequent the breaks in the geological se- 
ries may be in which they would fain bury their 
transition types, there are many points in the 
succession where the connection is perfectly dis- 
tinct and unbroken, and it is just at these points 
that new organic groups are introduced without 
any intermediate forms to link them with the 
preceding ones. In another series of papers, I 
shall endeavor to show the futility of the argu- 
ment so far as it is founded upon the imperfec- 
tion of the geological record. 
I would add one word upon the way in which 
this volume has been prepared, since it has some 
features requiring explanation, if not apology. 
These chapters’ were first embodied in a course 
