294 MISCELLANEOUS OBJECTIONS TO THE [Chap. VII 



upper side of the turbot, are due to the action of the 

 light. Here natural selection has probably come into 

 play, as well as in adapting the general shape of the body 

 of these fishes, and many other peculiarities, to their 

 habits of life. We should keep in mind, as I have be- 

 fore insisted, that the inherited effects of the increased 

 use of parts, and perhaps of their disuse, will be strength- 

 ened by natural selection. For all spontaneous varia- 

 tions in the right direction will thus be preserved; as 

 will those individuals which inherit in the highest de- 

 gree the effects of the increased and beneficial use of any 

 part. How much to attribute in each particular case to 

 the effects of use, and how much to natural selection, it 

 seems impossible to decide. 



I may give another instance of a structure which 

 apparently owes its origin exclusively to use or habit. 

 The extremity of the tail in some American monkeys 

 has been converted into a wonderfully perfect prehensile 

 organ, and serves as a fifth hand. A reviewer who 

 agrees with Mr. Mivart in every detail, remarks on this 

 structure: "It is impossible to believe that in any num- 

 ber of ages the first slight incipient tendency to grasp 

 could preserve the lives of the individuals possessing it, 

 or favour their chance of having and of rearing off- 

 spring." But there is no necessity for any such belief. 

 Habit, and this almost implies that some benefit great 

 or small is thus derived, would in all probability suffice 

 for the work. Brehm saw the young of an African 

 monkey (Cercopithecus) clinging to the under surface 

 of their mother by their hands, and at the same time 

 they hooked their little tails round that of their mother. 

 Professor Henslow kept in confinement some harvest 

 mice (Mus messorius) which do not possess a structu- 



