VARIABILITY OF TPIE SKELETON. 93 



near to the body, and sometimes very forward ; the other, 

 usually more restricted, is made in a direction perpendicular 

 to the former ; it is that which constitutes the stroke of the 

 wing. 



' Curvatures of different radii correspond, therefore, to these 

 two movements of unequal amplitude ; to the greater move- 

 ment of stretching and folding the wing a curvature of short 

 radius corresponds ; to the less extensive movement which 

 raises and lowers the wing during, flight, there is a corre- 

 sponding curved surface of very long radius. The result of 

 this is that the head of the humerus in birds assumes the 

 form of a very elongated ellipse, at the level of the articular 

 surface. 



But the movements of flight present in different species 

 great variations of amplitude. Birds which have sail-like 

 wings give but very small strokes with them, while the 

 pigeon, at the moment when it takes flight, strikes its wings 

 one against the other above and below, producing a clapping 

 noise, which is familiar to every one. 



To these variations in the extent of the movements corre- 

 spond varieties of surface in the head of the humerus, which 

 in birds with sail-like wings has a very elongated elliptical 

 surface ; but in the pigeon it tends to the circular form, and 

 very nearly attains it in the spheniscus, an aquatic bird found 

 in southern seas, and closely resembling the penguin. 



From all this we may gather, that in the form of the bony 

 structure, everything bears the trace of some external influ- 

 ence, and particularly of the function of the muscles. There 

 is not a single depression or projection in the skeleton, 

 the cause of which cannot be found in an external force, 

 which has acted on the bony matter, either to indent it, or 

 draw it forward. It was not, therefore, a metaphorical exag- 

 geration to say, that the bone is subject, like soft wax, to all 

 the changes of form which external forces tend to impress 

 upon it ; and that, notwithstanding its extreme hardness, it 

 resists less than the most supple tissues the efforts which tend 

 to change its form. 



And will this new form, acquired by means of function, 

 disappear with the individual? Will he not transmit even 



