CONTENTS. vii 



tion — Correlated variation-^Compensation and economy of 

 growth — False correlations — Multiple, rudimentary, and lowly 

 organised structures variable — Parts developed in an unusual 

 manner are highly variable ; specific characters more variable 

 than generic: secondary sexual characters variable — Species 

 of the same genus vary in an analogous manner — Reversions to 

 long-lost characters — Summary .... Page 164 



CHAPTER VI. 



DIFFICULTIBS OF THE TflBOBY. 



Difficulties of the theory of descent with modification — Absence or 

 rarity of transitional varieties — Transitions in habits of life — 

 Diversified habits in the same species — Species with habits 

 widely different from those of their allies — Organs of extreme 

 perfection — Modes of transition — Cases of difficulty — Natura 

 non facit saltum — Organs of small importance — Organs not in 

 all cases absolutely perfect — The law of Unity of Type and of 

 the Conditions of Existence embraced by the theory of Natu- 

 ral Selection 207 



CHAPTER VII. 



MISCELLANEOUS OBJECTIONS TO THE THEORY OF NATURAL 

 SELECTION. 



Longevity— Modifications not necessarily simultaneous— Modifica- 

 tions apparently of no direct service- Progressive development 

 —Characters of small functional importance, the most con- 

 stant—Supposed incompetence of natural s^lection to account 

 for the incipient stages of useful structures— Causes which in- 

 terfere with the acquisition through natural selection of useful 

 structures— Gradations of structure with changed functions— 

 Widely different organs in members of the same class, de- 

 veloped from one and the same source — Reasons for disbeliev- 

 ing in great and abrupt modifications .... 263 



