762 THE POPULAR SCIENCE MONTHLY. 



ately greater size of all its components — bones, muscles, arteries, veins, 

 etc. But though in cases like this, the co-operative parts forming some 

 large complex part may be expected to vary together, nothing implies 

 that they necessarily do so ; and we have proof that in various cases, 

 even when closely united, they do not do so. An example is furnished 

 by those blind crabs named in the Origin of Species which inhabit 

 certain dark caves of Kentucky, and which, though they have lost 

 their eyes, have 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 man- 

 kind these have not varied together. During civilization the jaws 

 have decreased, but the teeth have not decreased in proportion ; and 

 hence that prevalent crowding of them, often remedied in childhood 

 by extraction of some, and in other cases causing that imperfect de- 

 velopment which is followed by early decay. But the absence of pro- 

 portionate 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 are appended 

 to the entry which identifies a Blenheim Spaniel's skull, the words — 

 "the teeth are closely crowded together," and to the entry concerning 

 the skull of a King Charles's Spaniel the words — "the teeth are closely 

 packed, p. 3 , is placed quite transversely to the axis of the skull." 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, a like 

 want of concomitant variation is manifested : the case being that of 

 the bull-dog, in the upper jaw of which also, " the premolars . . . are 

 excessively crowded, and placed obliquely or even transversely to the 

 long axis of the skull." * 



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

 variation in co-operative parts that are near together — if we do not 

 find it in parts which, though belonging to different tissues, are so 

 closely united as teeth and jaws — if we do not find it even when the 

 co-operative parts are not only closely united, but are formed out of 

 the same tissue, like the crab's eye and its peduncle ; what shall we 

 say of co-operative parts which, besides being composed of different 

 tissues, are remote from one another ? Not only are we forbidden to 



* 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 pecul- 

 iarity in this respect seems due to relative shortness of the upper jaw, giving the under, 

 htmg structure which, involving retreat of the nos-trils, enables the dog to continue 

 breathing while holding. 



