CHAP. XII.] THE DIRECTION OF DEVELOPMENT. 133 



separate parts of the organisms for the discharge of the 

 separate functions ; one set of organs for the nutritive or 

 assimilative function, another for the excretory, another 

 for the motor, another for the reproductive, and so on. To 

 use an expressive metaphor — and, indeed, more than a 

 metaphor, a real analogy — vital organization consists in 

 framing the various parts of the organism for the physio- Physiologi- 

 logical division of lalour; and those are the most highly ^^ jabour!'^ 

 organized beings in which the distinctness of tissues and 

 organs,' and consequently the physiological division of 

 labour between them, are carried the farthest. 



In the lowest organisms, as we have seen, every part 

 discharges all functions : there is no physiological division 

 of labour at all. And even in the highest it is not com- Never 

 plete : some of the organs have to a great extent the power pj^ete. *^''™' 

 of discharging the functions of others. In man, for in- 

 stance, the skin and the kidneys are to a great extent able 

 each to do the work of the other in separating water and 

 soluble substances from the blood ; and in some cases of 

 disease one excretory organ assumes the whole work of 

 another. This power, which is so utterly unlike anything 

 in human machinery, is no doubt in some way due to the 

 common origin of all organs and all tissues in the body in 

 the original homogeneous and structureless germ.^ 



But, as we have seen, the highest organization is that in 

 which the various parts are each most thoroughly fitted 

 for its own peculiar function, and where, consequently, 

 each part has least power to discharge any function, or do 

 any work, other than its own. It follows from this that 

 the highest organisms are those in which the combination Combined 

 of the action of the various parts is most perfect ; and in p^rts^ 

 them, also, the combination of the action of the various most per- 

 parts is most necessary. In other words : in the highest highest 

 organization the various parts minister to each other most organisms, 

 perfectly, and are least able to do without each other. In 

 such a low organism as a hydra, for instance, the mutual 

 dependence of parts is so slight, that if the animal be cut 

 in pieces, each part will become a perfect individual ; and 



1 Carpenter's Human Physiology, p. 37i. 



