THE INNER TISSUES OF ANIMALS. 305 



the functionally-determined modifications of muscle are in 

 heritable. 



Muscles disagree greatly in their tints: all gradations 

 between white and deep red being observable. Contrasts are 

 visible between the muscles of different animals, between the 

 muscles of the same animal at different ages, and between 

 different muscles of the same animal at the same age. 

 We will glance at the facts under these heads : noting under 

 each of them the connexion which here chiefly concerns 

 us that between the activity of muscle and its depth 

 of colour. The cold-blooded Vertebrata are, taken 



as a group, distinguished from the warm-blooded by the 

 whiteness of their flesh ; and they are also distinguished by 

 their comparative inertness. Though a fish or a reptile can 

 exert considerable force for a short time, it is not capable 

 of prolonged exertion. Birds and mammals show greater 

 endurance along with the darker-coloured muscles. If among 

 birds themselves or mammals themselves we make compari 

 sons, we meet with kindred contrasts especially between 

 wild and domestic creatures of allied kinds. Barn-door fowls 

 are lighter-fleshed than most untamed gallinaceous birds; 

 and among these last the pheasant, moving about but little, 

 is lighter-fleshed than the partridge and the grouse which are 

 more nomadic. The muscles of the sheep are not on the 

 average so dark as those of the deer; and it is said that 

 the flesh of the wild-boar is darker than that of the pig. 

 Perhaps, however, the contrast between the hare and the 

 rabbit affords, among familiar animals, the best example of 

 the alleged relation: the dark-fleshed hare having no retreat 

 and making wide excursions, while the white-fleshed rabbit, 

 passing a great part of its time in its burrow, rarely wanders 

 far from home. The parallel contrast between young 



and old animals has a parallel meaning. Veal is much 

 whiter than beef, and lamb 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 



