18 MORPHOLOGICAL DEVELOPMENT. 



ridge-tiles, 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 organisms. 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 structures. 

 The physiological units which we are obliged to assume as 

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

 be the possessors of those proclivities which result in 

 the structural arrangements of the organism. The assump- 

 tion 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 

 some degrees higher in complexity than those molecules of 

 nitrogenous colloidal substance into which organic matter is 

 resolvable; and we regard these very much more complex 

 molecules as having the implied greater instability, greater 

 sensitiveness to surrounding influences, and consequent 

 greater mobility of form. Such being the primitive physio- 

 logical units, organic evolution must begin with the formation 

 of a minute aggregate of them an aggregate showing vitality 

 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, 



