368 PHYSIOLOGICAL DEVELOPMENT. 



become less active decrease; then a notable difference of size 

 should exist between the muscles used for flight in birds that 

 fly much, and those in birds of an allied kind that fly little. 

 But, as we here see, this is not the true implication. The 

 change in such cases must be chiefly in vascularity and abun- 

 dance of contractile substance; and cannot be, to any great 

 extent, in bulk. For a bird to fly at all, its pectoral muscles, 

 bones of attachment, and all accompanying appliances, must 

 be kept up to a certain level of power. If the parts dwindle 

 much, the creature will be unable to lift itself from the 

 ground. Bearing in mind that the force which a bird ex- 

 pends to sustain itself in the air during each successive instant 

 of a short flight is, other things equal, the same as it ex- 

 pends in each successive instant of a long flight, we shall see 

 that the muscles employed in the two cases must have some- 

 thing like equal intensities of contractile power ; and that the 

 structural differences between them must have relation mainly 

 to the lengths of time during which they can continue to re- 

 peat contractions of like intensity. That is to say, while the 

 power of flight is retained at all, the muscles and bones can- 

 not greatly dwindle ; but the dwindling, in birds whose flights 

 are short or infrequent or both, will be in the reserve stock 

 of the substance that is incapacitated by action, or in the 

 appliances that keep the apparatus in repair, or in both. 

 Only where, as in the struthious birds, the habit of flight is 

 lost, can we expect atrophy of all the parts concerned in 

 flight ; and here we find it. 



Are such differentiations among the muscles functionally 

 produced? or are they produced by the natural selection of 

 variations distinguished as spontaneous? We have, I think, 

 good grounds for concluding that they are functionally pro- 

 duced. We know that in individual men and animals, the 

 power of sustained action in muscles is rapidly adaptable to 

 the amount of sustained action required. We know that 

 being "out of condition," is usually less shown by the inability 

 to put out a violent effort than by the inability to continue 



