366 PHYSIOLOGICAL DEVELOPMENT. 



expend more muscular force than their sedate dams ; yet the 

 meaning of the contrast is really as alleged. For in conse- 

 quence 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 much more 

 in muscular effort than do those of a young 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, namely 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 which exist between 

 the active and inactive muscles of the same animal. Between 

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

 ence of colour is familiar; and we know that fowls exercise 

 their leg-muscles much more than the muscles which move 

 their wings. Similarly 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 constantly as their legs, little or no difference is visible 

 between the colour of these two groups of muscles. Special 

 contrasts like these do not, however, exhaust the proofs; for 

 there is a still more significant general contrast. The 

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

 is the darkest of all muscles. 



The connexion of phenomena thus shown in so many ways, 

 implies that the bulk of a muscle is by no means the sole 

 measure of the quantity of force it can evolve. It would seem 

 that, other things equal, the depth of colour varies with the 

 constancy of action ; while, other things equal, the bulk varies 

 with the amount of force that has to be put forth upon occa- 



