PROGRESSION. 1"27 



LECTURE XL 

 PROGRESSION. 



Locomotion implies vaguely the act or power of moving from 

 place to place, and is equally applicable to animals with and 

 without feet ; PROGRESSION carries in its meaning the notion of 

 feet, and signifies stepping forward ; RETRO -GRESSION being its 

 antagonist term — the word we use for stepping backward. 



From the bare facts of quadrupeds being known to be the fleet- 

 est of the creatures that move upon the earth's surface, those that 

 are capable of the greatest feats of saltation, as well as of the 

 greatest speed and endurance in the course, we might safely infer 

 that four legs constituted a number better calculated for progression 

 than any other. Men can run and jump with, considering they have 

 but two legs, surprising effect ; but neither in the act of progres- 

 sion nor in that of saltation can they compete with certain qua- 

 drupeds ; neither are many-footed creatures — centipedes, as some 

 of them are denominated — to be compared in these respects with 

 quadrupeds, or even, indeed, with bipeds : the number four ap- 

 pearing, in relation to legs, to be that which most happily answers 

 the purposes of succession in stepping and propulsion, as well as 

 for that continual shifting of the centre of gravity which necessarily 

 takes place in the transportation of the body. 



THE CENTRE OF GRAVITY, in a quadruped standing with its 

 legs in their natural position, will be found to fall anterior to a 

 point equidistant from each of the four feet, owing to the pre- 

 ponderance forward of the head and neck : the precise point upon 

 which the line of gravitation will fall, it will be difficult or impos- 

 sible to determine, that in some inconsiderable degree continually 

 varying from the circumstance of the perpetual changes in the 

 erection and declination of the head and neck, not to notice the 

 unimportant fluctuations that may be caused in it by the move- 

 ments of respiration and by the constant shifting of place of the 

 viscera. The line of gravitation will be liable, however, to undergo 

 variations of some consequence from the imposition of weight upon 

 the animal's back, and these will be found to be of a nature cor- 

 respondent with the situation of the weight imposed, its bulk or 



