358 PHYtilOLOGlCAL DEVELOPMENT. 



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 oc- 

 casion. These of course are approximate relations. More 

 correctly we may say that the actions of pale muscles are 

 either relatively feeble though frequent (as in the massive 

 fldiiks of a fish), or relatively infrequent though strong (as in 

 the pectoral muscles of a common fowl) ; while the actions of 

 dark muscles are both frequent and strong. Some such dif- 

 ferentiation may be anticipated by inference from the respec- 

 tive physiological requirements. A muscle which has upon 

 occasion to evolve considerable force, but which has thereafter 

 a loiig period of rest during which repair may restore it to 

 efficiency, requires neither a large reserve of the contrac- 

 tile substance that is in some way deteriorated by action, 

 nor highly-developed appliances for bringing it nutri- 

 tive materials and removing efiete products. Where, con- 

 trariwise, an exerted muscle that has undergone much 

 molecular change in evolving mechanical force, has soon again 

 to evolve much mechanical force, and so on continually ; it 

 is clear that either the quantity of contractile substance 

 present must be great, or the apparatus for nutrition and 

 depuration must be very efficient, or both. Hence we may 

 look for marked unlikenesses of minute structure between 

 muscles that are markedly contrasted in activity. And we may 

 suspect that these conspicuous contrasts of colour between 

 active and inactive mxiscles, are due to these implied diffe- 

 rences of minute structure — partly diifereuces between the 

 numbers of blood-vessels and partly differences between the 

 quantities of sarcous matter. 



Here, then, we have a key to the apparent anomaly above 

 hinted at — the maintenance of bulk by certain muscles which 

 have been rendered comparatively inactive by changed habits 

 of life. That the pectoral muscles of those domestic birds 



