THE INNER TISSUES OF ANIMALS. 357 



far from home. The parallel contrast between 



young and old animals has a parallel meaning. Veal is 

 much whiter than beef, and iamb is of lighter colour than 

 mutton. Though at first sight these facts may. not seem 

 to furnish confirmatory evidence, since lambs in their play 

 appear to expend more muscular force than their sedate 

 dams ; yet the meaning of the contrast is really as alleged. 

 For in consequence of the law that the strains which animals 

 have to overcome, increase as the cubes of the dimensions, 

 while their powers of overcoming them increase only as the 

 squares (§ 46), the movements of an adult animal cost very 

 much more in muscular efibrt than do those of a 3'oung 

 animal : the result being that the sheep and the cow exercise 

 their muscles more vigorously in their quiet movements, than 

 the lamb and the calf in their lively movements. It may be 

 added as significant, that the domestic animal in which no 

 very marked darkening of the flesh takes place along with 

 increasing age, namel}^ the pig, is one which, ordinarily kept 

 in a sty, leads so quiescent a life that the assigned cause of 

 darkening does not come into action. But perhaps 



the most conclusive evidences are the contrasts that exist 

 between the active and inactive muscles of the same animal. 

 Between the leg- muscles of fowls and their pectoral muscles, 

 the difference of colour is familiar ; and we know that fowls 

 exercise their leg-muscles much more than the muscles which 

 move their wings. Similarl}' in the turkey, in the guinea 

 fowl, in the pheasant. And then, adding much to the force of 

 this evidence, we see that in partridges and grouse, which 

 belong to the same order as our domestic fowls, but use their 

 wings as habitually as their legs, little or no difference is 

 visible between the colours of these two groups of muscles. 

 Special contrasts like these do not, however, exhaust the 

 proofs ; for there is a still more significent general contrast. 

 The muscle of the heart, which is the most active of all 

 muscles, is the darkest of all muscles. 



The connection of phenomena thus shown in so many ways, 



