XII.] 



THE DIEECTION OF DEVELOPMENT. 135 



moderately well. In the living organism, and in human Efficiency 

 society alike, the purpose of division of labour is to increase ancrall 

 the efficiency of every member ; the purpose of combiiia- members 

 tion, or centralization, is to increase the efficiency of their by division 

 combined action : each of these— division of function and °^J|^^°^"' 

 combination of action — is necessary to the other, and is bination. 

 involved in the other. 



From physiological division of labour follows definiteness Definite- 



ll p a a n 



of the number, position, and form of parts. Where every j.gs,^i^. ^f 

 part has its perfectly definite function, it follows that the division of 

 number, form, and position of the parts must be definite. 

 We find that in the lower organisms the number of the 

 parts, especially, is often very indefinite ; but as we ascend 

 in the scale of organization this characteristic disappears. 

 Nothina;, for instance, can well be less definite than the 

 number of leaves on a bough ; but in the flower, which is Leaves and 

 by far the most highly organized part of the plant, the 

 number of the petals and of all the other parts, in many 

 important classes at least, is perfectly definite. And in 

 worms and millepedes the number of segments is inde- Worms, 

 finite, being variable between different individuals of the ™g^g'g' ^^^ 

 same species ; but among insects, crabs, and spiders, which insects. 

 are more highly organized on the same fundamental plan, 

 the number of segments is constant. 



What precedes may be summed up by saying, that the Summary, 

 higher the organization the more complete are the physio- 

 logical division of labour, the physiological centralization, 

 and the mmphological definiteness. 



It would be impossible to give a fu.ll account of aU the 

 tissues and organs which become distinct from each other 

 in the development of aU the various classes of organisms, 

 without writing a complete treatise on comparative histo- 

 logy and anatomy. But this is the place to describe in 

 outline the chief distinctions of the functions, and of the 

 corresponding organs, which we find in all organic forms, 

 except the very lowest. 



The first and the only universal distinction of organs is Separation 

 tha.t produced by the gelatinous matter which constitutes ° "^ ^^^^ 



