428 THE POPULAR SCIENCE MONTHLY. 



ever, we trace out the gradual development of the more complex ani- 

 mal forms, we see coming forward into greater and greater prominence 

 the arrangement of the tissues in definite ways to secure mechanical 

 ends. Thus the entire body acquires particular shapes, and parts of 

 the body are built up into mechanisms, the actions of which are to the 

 advantage of the individual. Into the composition of these mechan- 

 isms, or " organs," the active, fundamental tissues, as well as the pas- 

 sive or indifferent tissues, enter ; and the working of each mechanism, 

 the function of each organ is dependent partly on the mechanical con- 

 ditions offered by the passive elements, partly on the activity of the 

 active elements. The vascular mechanism, of which we have just 

 spoken, is such a mechanism. Similarly, the urgent necessity for the 

 access of oxygen to all parts of the body has given rise to a compli- 

 cated respiratory mechanism ; and the needs of copious alimenta- 

 tion, to an alimentary or digestive meclianism. 



Further, inasmuch as muscular movement is one of the chief ends, 

 or the most important means to the chief ends of animal life, we find 

 the animal body abounding in motor mechanisms, in which the prime 

 mover is muscular contraction, while the machinery is supplied by 

 complicated arrangements of muscles with such indifferent tissues as 

 bone, cartilage, and tendon. In fact, the greater part of the animal 

 body is a collection of muscular machines, some serving for locomo- 

 tion, others for special manoeuvres of particular members and parts, 

 others as an assistance to the senses, and yet others for the produc- 

 tion of voice, and, in man, of speech. 



Lastly, the simple automatism of the amoeba, with its simple re- 

 sponses to external stimuli, is replaced in the higher animals by an 

 exceedingly complex volition, affected in multitudinous ways by in- 

 fluences from the world without ; and there is a correspondingly com- 

 plex central nervous system. And here we meet with a new form of 

 differentiation unknown elsewhere. While the contractility of the 

 amoebal protoplasm differs at the most but slightly from the contrac- 

 tility of the vertebrate striated muscle, there is an enormous differ- 

 ence between the simple irritability of the amoeba and the complex 

 action of the vertebrate nervous system. Excepting the nervous or 

 irritable tissues, the fundamental tissues have in all animals exactly 

 the same properties, being, it is true, more acute and perfect in one 

 than in another, but remaining fundamentally the same. The ele- 

 mentary muscular fibre of a mammal is at most a mass of but slightly 

 differentiated protoplasm, forming a whole physiologically continuous, 

 and in no way constituting a mechanism. Each fibre is a counterpart 

 of all others ; and the muscle of one animal differs from that of anoth- 

 er in such particulars only as are wholly subordinate. In the nervous 

 tissues of the higher animal, on the contrary, we find properties un- 

 known to those of the lower ones ; and, in proportion as we ascend 

 the scale, avc observe an increasing differentiation of the nervous sys- 



