14 DARWINISM AND EVOLUTION. 



needed for its support ; and these, in turn, must be furnished not 

 only with more blood but with an augmented nerve-force, while 

 the bony ridges to which the muscles are attached must grow 

 stronger and more prominent. 



Now, on the one hand, if the total quantity of the blood and 

 nerve-force of the body remains the same, some organs must 

 be supplied at the expense of others. 



On the other hand, if the total quantity of blood is increased, 

 more food must be taken, the digestive functions must rise in 

 activity, and, as actually occurs during the gestation or pregnancy 

 of mammals, the heart which drives the increased quantity of 

 blood must itself grow larger and more powerful. 



But with a larger and more powerful heart, and with an increased 

 quantity of blood, other organs will be stimulated ; and so waves 

 of change, correlated coincident variations, pervade the entire 

 system in every conceivable combination. 



The operation of the same law is seen in plants. In composites, 

 the flowers of the circumference, partly as the result of their 

 position, have their corollas much more developed than those of 

 the centre ; and sometimes the seeds of the circumference differ 

 greatly from those of the centre, while occasionally the reproductive 

 organs of the circumferential flowers are quite abortive. 



9. The ninth proposition is that structures are nourished in 

 proportion to their use. Witness the blacksmith's arm, and the 

 udders of cows that are habitually milked. Darwin found that in 

 the domestic duck the bones of the wing weigh less, and the 

 bones of the leg more, in proportion to the whole skeleton, than 

 do the same bones in the wild-duck ; a difference to be attributed 

 to the domestic duck flying much less and walking more than its 

 wild parents. 



The effects of disuse are shown in the eye of moles, and of a 

 few burrowing rodents, which, in some cases, is of minute size, 

 and in other cases is quite covered by skin and fur. In certain 

 cave-crabs, too, the stalk for the eye remains, though the eye is 

 gone. 



Parasites present further examples. Some of the Hippoboscidce, 

 especially those that infest birds like lice, have a pair of slender 

 wings, but from long-continued inaction they are powerless for 

 flight ; the four wings possessed by the ancestors of the flea are 

 now represented by as many minute scales ; and there are some 

 intestinal worms that have no organs left except those that are 

 necessary for growth and reproduction. 



From the foregoing nine propositions, continues Prof. Fiske, 

 each one of which is indisputably true, it is an inevitable corollary 

 that changes thus set up, and complicated, must eventually alter 

 the specific character of any given group of organisms. In other 

 words, it follows that the descendants of some common ancestor 



