356 PHYSIOLOGICAL DEVELOPMENT. 



rentiated contractile tissues exhibit. These are the rela- 

 tions between muscular exercise, muscular power, and mus- 

 cular structure ; and it is the more needful for us here to 

 notice them because of certain anomalies they present, 

 which, at first sight, seem inconsistent with the belief that 

 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, be- 

 tween 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 con- 

 cerns 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 en- 

 durance along with darker- coloured muscles. If among birds 

 themselves or mammals themselves we make comparisons, 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 



