THE MORPHOLOGICAL COMPOSITION OF PLANTS. 11 



do not result from combination or metamorphosis of bricks, 

 but are made directly out of the original clay. And of like 

 natures are the criticisms which must be passed on the 

 generalization, that cells are the morphological units of or- 

 ganisms. To continue the simile, the truth turns out to 

 be, that the primitive clay or protoplasm out of which 

 organisms are built, may be moulded either directly, or 

 with various degrees of indirectness, into organic struc- 

 tures. The physiological units which we are obliged to as- 

 sume as the components of this protoplasm, must, as we have 

 seen, be the possessors of those complex polarities which re- 

 sult in the structural arrangements of the organism. The 

 assumption of such structural arrangements may go on, and, 

 in many cases, does go on, by the shortest route ; without the 

 passage through what we call metamorphoses. But where 

 such structural arrangements are reached by a circuitous 

 route, the first stage is the formation of these small aggre- 

 gates, which, under the name of cells, are currently regarded 

 as morphological units. 



The rationale of these truths appears to be furnished by the 

 hypothesis of evolution. We set out with molecules one 

 degree higher in complexity than those molecules of nitro- 

 genous colloidal substance into which organic matter is 

 resolvable ; and we regard these somewhat more complex mo- 

 lecules as having the implied greater instability, greater sen- 

 sitiveness to surrounding influences, and consequent greater 

 mobility of form. Such being the primitive physiological 

 units, organic evolution must begin, with the formation of a 

 minute aggregate of them — an aggregate showing vitality 

 only by a higher degree of that readiness to change its form 

 of aggregation, which colloidal matter in general displays ; 

 and by its ability to unite the nitrogenous molecules it meets 

 with, into complex molecules like those of which it is com- 

 posed. Obviously, the earliest forms must have been minute ; 

 since, in the absence of any but diffused organic matter, no 

 form but a minute one could find nutriment. Obviously, too, 



