760 THE POPULAR SCIEXCE MONTHLY. 



ments by which, in thousands of years, this reduction has been effect- 

 ed, could have given to an individual in which it occurred, such ad- 

 vantage as would cause his survival, either through diminished cost of 

 local nutrition or diminished weight to be carried. I did not then 

 exclude, as I might have done, two other imaginable causes. It may 

 be said that there is some organic correlation between increased size 

 of brain and decreased size of jaw : Camper's doctrine of the facial 

 angle being referred to in proof. But this argument may be met by 

 pointing to the many examples of small-jawed people who are also 

 small-brained, and by citing not infrequent cases of individuals re- 

 markable for their mental powers, and at the same time distinguished 

 by jaws not less than the average but greater. Again, if sexual selec- 

 tion be named as a possible cause, there is the reply that, even suppos- 

 ing such slight diminution of jaw as took place in a single generation 

 to have been an attraction, yet the other incentives to choice on the 

 part of men have been too many and great to allow this one to weigh 

 in an adequate degree ; while, during the greater portion of the period, 

 choice on the part of women has scarcely operated : in earlier times 

 they were stolen or bought, and in later times mostly coerced by par- 

 ents. Thus, reconsideration of the facts does not show me the inva- 

 lidity of the conclusion drawn, that this decrease in size of jaw can 

 have had no other cause than continued inheritance of those diminu- 

 tions consequent on diminutions of function, implied by the use of 

 selected and well-prepared food. Here, however, my chief purpose is 

 to add an instance showing, even more clearly, the connexion between 

 change of function and change of structure. This instance, allied in 

 nature to the other, is presented by those varieties, or rather sub- 

 varieties, of dogs, which, having been household pets, and habitually 

 fed on soft food, have not been called on to use their jaws in tearing 

 and crunching, and have been but rarely allowed to use them in catch- 

 ing prey and in fighting. No inference can be drawn from the sizes 

 of the jaws themselves, which, in these dogs, have probably been 

 shortened mainly by selection. To get direct proof of the decrease of 

 the muscles concerned in closing the jaws or biting, would require a 

 series of observations very difficult to make. But it is not difficult to 

 get indirect proof of this decrease by looking at the bony structures 

 with which these muscles are connected. Examination of the skulls of 

 sundry indoor dogs contained in the Museum of the College of Sur- 

 geons, proves the relative smallness of such parts. The only pug- 

 dog's skull is that of an individual not perfectly adult ; and though its 

 traits are quite to the point they cannot with safety be taken as evi- 

 dence. The skull of a toy-terrier has much restricted areas of inser- 

 tion for the temporal muscles ; has weak zygomatic arches ; and has 

 extremely small attachments for the masseter muscles. Still more 

 significant is the evidence furnished by the skull of a King Charles's 

 spaniel, which, if we allow three years to a generation, and bear in 



