XIV NOTES BY THE EDITOR. 



is proportional to its weight, that is the number of muscular fibres in 

 its construction. 



2d. The mean lengths of the different muscles employed at each 

 point are proportional to the perpendicular let fall from the center of 

 motion of the joint upon the direction in which the muscles act. 



He considers that the 2d postulate is supported by these three 

 considerations : (1) The distance through which the point of applica- 

 tion of a muscle is moved by its contraction, is proportional to the 

 mean length of the muscle. (2) It is geometrically evident that the 

 perpendiculars let fall on the directions of the muscles, are propor- 

 tional to the spaces moved through by their points of application. 

 (3) The Divine contriver of the joint has made a perfect mechanism, 

 and therefore employs a minimum expenditure of force. 



Natural Selection. Mr. Wallace, the English Naturalist, in a 

 paper recently read before the Anthropological Society, arrives at the 

 following conclusions, which help to account for the variation and 

 transmutation of species : (1.) Peculiarities of every kind are more 

 or less hereditary. (2.) The offspring of every animal vary more or 

 less in all parts of their organization. (3.) The universe in which 

 these animals live is not absolutely invariable. (4.) The animals in 

 any country (those at least which are not dying out) must at each suc- 

 cessive period be brought into harmony with the surrounding condi- 

 tions. These are all the elements required for change of form and 

 structure in animals, keeping exact pace with changes of whatever 

 nature in the surrounding universe. Such changes must be slow, for 

 the changes in the universe must be very slow ; but just as these slow 

 changes become important, when we look at results, after long periods 

 of action, as we do when we perceive the alterations of the earth's 

 surface during geological epochs : so the parallel changes in animal 

 form become more and more striking, according as the time they have 

 been going on is great, as we see when we compare our living ani- 

 mals with those which we disentomb from each successively older 

 geological formation. 



