860 Dynamic Theory. 



ments to the forces which work them. These adaptations are in some 

 cases so striking as to appear purposive. Take, for example, the adjust- 

 ments of the ear-drum, and other parts of the ear ; the otoliths, the 

 eustachian tube, the chromatic scale of the arches of corti, &c ; the re- 

 tina with its rods and cones, the lenses, the muscles for moving the ball, 

 the pulley for altering the direction of the pull of the superior oblique 

 muscle of the eye, &c. ; the synovial lubricating arrangements for the 

 joints, the sphincter muscles, the heart, the brain, the teeth, in short, 

 every part concerned in motion, or related to movement. 



There is not the slightest doubt, that all these adjustments and arrange- 

 ments are the result of the automatic action of various modes of ener- 

 getic movement. Even their use is in some cases automatic, and often 

 beyond the purposive control. That remarkable " pulley " muscle, the 

 superior oblique, is an involuntary muscle, and its contractions wholly 

 automatic. 



In what does the difference consist between these natural contrivances, 

 which have been built up through use without the intervention of a pre- 

 conceived purpose, and those artificial contrivances, such as a beaver's 

 dam, a spider's web, a bee's honey-comb, a birds' nest, a man's house or 

 locomotive ? 



The natural contrivances are self-reproductive. Generation succeeds 

 generation, repeating practically the same physical forms for ages. Even 

 in the minutest details, they are usually liable only to extremely slow 

 changes. 



The products of these several parts, whether they be mechanical 

 movements or sensations of some sort, likewise remain constant one 

 generation after another, and they may be supposed to become changed 

 only after some change has taken place in the machinery for their pro- 

 duction. 



Artificial works, although not self-reproductive, are also, nevertheless, 

 produced in many cases without material change for a great many suc- 

 cessive generations. Although no two bird's nests are precisely alike in 

 details, yet for a thousand generations the nests of any particular species 

 continue to be repeated after practically the same plan. The same is 

 true of the beaver dam, the honej r comb, the spider's web, and of ninety- 

 nine out of every hundred of the contrivances of man. These artificial 

 works, depending on the brain for their execution, remain as a constant 

 production so long as the brain continues to be handed down without 

 change from generation to generation. 



We cannot fail to see the parallel here between what we call artificial 

 productions and natural secretions. Tears, synovia, mucus, milk, urine, 

 &c. , are called secretions. They are articles of a foreign non-vital char- 

 acter produced by the action of certain organs called glands. They are 



