THE FACTORS OP ORGANIC EVOLUTION. 13 



not lost the foot-stalks which carried their eyes. In 

 describing the varieties which have been produced by 

 pigeon-fanciers, Mr. Darwin notes the fact that along with 

 changes in length of beak produced by selection, there have 

 not gone proportionate changes in length of tongue. Take 

 again the case of teeth and jaws. In mankind these have 

 not varied together. During civilization the jaws have 

 decreased, but the teeth have not decreased in propor 

 tion; and hence that prevalent crowding of them, often 

 remedied in childhood by extraction of some, and in 

 other cases causing that imperfect development which is 

 followed by early decay. But the absence of proportionate 

 variation in co-operative parts that are close together, and 

 are even bound up in the same mass, is best seen in those 

 varieties of dogs named above as illustrating the inherited 

 effects of disuse. We see in them, as we see in the human 

 race, that diminution in the jaws has not been accompanied 

 by corresponding diminution in the teeth. In the catalogue 

 of the College of Surgeons Museum, there is appended to 

 the entry which identifies a Blenheim Spaniel s skull, the 

 words &quot;the teeth are closely crowded together,&quot; and to 

 the entry concerning the skull of a King Charles s Spaniel 

 the words &quot; the teeth are closely packed, p. 3, is placed 

 quite transversely to the axis of the skull.&quot; It is further 

 noteworthy that in a case where there is no diminished use 

 of the jaws, but where they have been shortened by selection, 

 r a like want of concomitant variation is manifested : the case 

 being that of the bull-dog, in the upper jaw of which also, 

 &quot; the premolars . . . are excessively crowded, and placed 

 obliquely or even transversely to the long axis of the skull. &quot;* 

 If, then, in cases where we can test it, we find no con- 



* It is probable that this shortening has resulted not directly but indirectly, 

 from the selection of individuals which were noted for tenacity of hold ; for 

 the bull-dog s peculiarity in this respect seems due to relative shortness of 

 the upper jaw, giving the underhung structure which, involving retreat of 

 the nostrils, enables the dog to continue breathing while holding. 



