THE OUTER AND INNER TISSUES OF PLANTS. 227 



the other, the contrast is due to the unlike actions to which 

 the parts are subject. Given an originally -homogenous 

 portion of protoplasm, and it follows from the general laws 

 of Evolution (First Principles, §§ 109 — 115) first, that it must 

 lose its homogeneity, and, second, that the leading dissimila- 

 rities must arise between the parts most-dissimilarly con- 

 ditioned — that is, between the outside and the inside. The 

 exterior must bear amounts and kinds of force unlike the 

 amounts and kinds which the interior bears; and from 

 the persistence of force it follows inevitably that unlike 

 effects must be wrought on them — they must be differen- 

 tiated. 



What is the limit towards which the differentiation 

 tends ? We have seen that the re-distribution of matter 

 and motion whence, under certain conditions, evolution 

 results, can never cease until equilibrium is reached — proxi- 

 mately a moving equilibrium, and finally a complete equi- 

 librium (First Principles, §§ 130 — 135). Hence, the diffe- 

 rentiation must go on until it establishes such differences in 

 the parts as shall balance the differences in the forces acting 

 on them. When dealing with equilibration in general, we 

 saw that this process is what is called adaptation (First 

 Principles, § 133) ; and, more recentty, we saw that by it the 

 totality of functions of an organism is brought into cor- 

 respondence with the totality of actions affecting it (§§ 159 

 ■ — 163). Manifestly in this case, as in all others, either 

 death or adjustment must eventually result. A force falling 

 on one of these minute aggregates of protoplasm, must ex- 

 pend itself in working its equivalent of change. If this 

 force is such that in expending itself it disturbs be} r ond 

 rectification the balance of the organic processes, then the 

 aggregate is disintegrated or decomposed. But if it does 

 not overthrow that moving equilibrium constituting the life 

 of the aggregate, then the aggregate continues in that modi- 

 fied form produced by the expenditure of the force. Thus, 

 by direct equilibration, continually furthered by indirect 



