TnAP. IV.] MANNER OF DEVELOPMENT. 145 



with other structures. The tail almost always tapers 

 toward the end, whether it be long or short ; and this, I 

 presume, results from the atrophy, through disuse, of the 

 terminal muscles, together with their arteries and nerves, 

 leading to the atrophy of the terminal bones. With re- 

 spect to the os coccyx, which in man and the higher apes 

 manifestly consists of the few basal and tapering segments 

 of an ordinary tail, I have heard it asked how could these 

 have become completely embedded within the body ; but 

 there is no difficulty in this respect, for in many monkeys 

 the basal segments of the true tail are thus embedded. 

 For instance, Mr. Marie informs me that in the skeleton ot 

 a not full-grown 3Iacacus inornatus, he counted nine or 

 ten caudal vertebrae, which altogether were only 1.8 inch 

 in length. Of these the three basal ones appeared to have 

 been embedded ; the remainder forming the free part of 

 the tail, which was only one inch in length, and half an 

 inch in diameter. Here, then, the three embedded caudal 

 vertebrae plainly correspond with the four coalesced ver- 

 tebrae of the human os coccyx. 



I have now endeavored to show that some of the most 

 distinctive characters of man have in all probability been 

 acquired, either directly, or more commonly indirectly, 

 through natural selection. "We should bear in mind that 

 modifications in structure or constitution, which are of no 

 service to an organism in adapting it to its habits of life, 

 to the food which it consumes, or passively to the sur- 

 rounding conditions, cannot have been thus acquired. 

 We must not, however, be too confident in deciding what 

 modifications are of service to each being : we should re- 

 member how little we know about the use of many parts, 

 or what changes in the blood or tissues may serve to fit 

 an organism for a new climate or some new kind of food. 

 Nor must we forget the principle of correlation, by which, 



