TRACHEATES 2 I I 



present form by a continuous selection of the shortest- 

 legged pups. In its variations there were always 

 corresponding modifications of the other organs, such 

 as, broader paws, thicker legs, and changes in the 

 bony structure and length of the body. The latter 

 feature is absolutely necessary to secure mobility in a 

 low animal, as we see in the case of all short-legged 

 or legless animals, like the marten, the lizard, or the 

 serpent. Just as, in the selection of the dachshund, 

 all the corresponding variations appeared quite spon- 

 taneously, without the vital activity of the developing 

 variety being drawn upon, that might also happen in 

 the parallel case of natural selection, and therefore in 

 the selection of the giant antlers of our stag. We must 

 not forget, moreover, that an organism is a harmonious 

 whole, in which continuous selection has brought about 

 an ever improving co-adaptation and co-operation of 

 the various parts. Hence, when the variation of a 

 longer bone occurs in a leg, the corresponding muscles, 

 blood-vessels, and nerves are also usually longer. It 

 has even been discovered that when fly-maggots are 

 kept without food, the flies that issue from them are 

 smaller than usual, but have a complete harmony of the 

 various organs. Selection has led to this harmony, and 

 it preserves and increases it ; every organism that lacks 

 it will be crushed out as a cripple. Moreover, natural 

 selection can often develop similar organs in different 

 directions. We see this in the legs of the leaping 

 mammals, in which the hind legs are much longer than 

 the front. 



Further, co-adaptations may often be brought about 



