PHYSIOLOGICAL ILLUSTRATIONS. 41 



a new faculty, or the addition of a new organ, brings with it a corresponding change 

 in the arrangement of the whole plan. An engineer, who wishes to adapt a machine 

 to the execution of some new task, alters every part, no matier how remote it may be 

 from the acting point, until every wheel and lever executes its work co-ordinately with 

 all the others ; the prime mover remains unchanged, though the general character of the 

 machine may have undergone a renovation ; and as all machines, no matter of how 

 many parts they are composed, nor of how many wheels they consist, nor how intri- 

 cate soever may be their resulting motions, may have their power reduced to and rep- 

 resented by a simple lever, so also organic functions, though often brought about by 

 highly complex arrangements, find simple representatives in the lower orders of life. A 

 concentration, or a development of any organ, is often demanded by change in a re- 

 mote part of the fabric, when even the connexion may not be very evident Animals, 

 consisting simply of digesting cavities, require no vascular system for propelling or con- 

 taining a nutritious fluid ; they are not in need of separate tissues devoted to its oxyge- 

 nation, nor of an insulated respiration, nor do they demand distinct biliary organs ; when 

 the nutritious chyle is produced in the stomach of zoophytes, it finds its way into the 

 inter-cellular spaces, and there circulates without vessels, undergoing through the external 

 tegument the chemical changes. In many insect tribes, the bronchial tubes are spent 

 upon the peritoneum, and respiration takes place directly upon the alimentary canal. 

 With modification of functions, change of external figure is always involved ; and as 

 these progress together, systems of living things are constructed, referrible to one com- 

 mon original type. It is thus, in the echinodermata, we trace up successive steps, from 

 the sea urchin to the asterias, and from that to the pentacrinite ; a development of the 

 same parts of the structure continually taking effect, until the extremes bear no sort of 

 resemblance to each other. 



134. Had the production of living things been effected by the operation of second 

 causes, we might look, with LAMARCK, for some law of successive development w_hich 

 should contain the origin of each order and species. We might regard the rudimentary 

 teeth of whales, or the sub-cutaneous feet of the ophidia, as abortive results of such a law. 

 Considering the brain as a development of the spinal axis, we might trace in the form 

 of the cranial bones a development of a system of vertebrae, brought about as a conse- 

 quence of the very same laws. We might run a parallel of analogies between the crus- 

 taceous and vertebrated animals, and exogenous and endogenous plants ; we might take 

 the cephalopodous mollusks as furnishing the first rudiments of an internal skeleton, and 

 trace its increasing complexity to meet certain ends until its perfect development in the 

 mammalia. In this latter class, we might dwell upon the uniform existence of seven 

 cervical vertebrae, as giving evidence of a persistence in the plan of structure in spe- 

 cies so remote from each other as the camelopard, the whale, and the mole. Parting 

 from the dorsal vessel of insects, the first rudiments of an aorta, we might follow out 

 the complications of the higher arterial systems. In all the varieties of respiration, 

 whether aquatic, aerial, or mixed, we might see the reproduction of one original chem- 

 ical design, and in every instance of a concentration of machinery or functions, we 

 ini^ht find an impress of the action of external formative agents. 



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