THE INNER TISSUES OF ANIMALS. 367 



sion. 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 

 flanks 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 long period of rest during which repair may restore it to 

 efficiency, requires neither a large reserve of the contractile 

 substance that is in some way deteriorated by action, nor 

 highly-developed appliances for bringing it nutritive mate- 

 rials and removing effete products. Where, contrariwise, an 

 exerted muscle which has undergone much molecular change 

 in evolving much 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 which are 

 markedly contrasted in activity. And we may suspect that 

 these conspicuous contrasts of colour between active and 

 inactive muscles, are due to these implied differences of 

 minute structure: partly differences between the numbers 

 of blood-vessels and partly differences between the quantities 

 or qualities 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 

 which fly but little, have not dwindled to any great extent, 

 has been thought a fact at variance with the conclusion that 

 functionally-produced adaptations are inheritable. It has 

 been argued that if parts which are exercised increase, not 

 only in the individual but in the race, while parts which 



