252 PHYSIOLOGICAL DEVELOPMENT. 



gates of the second order a like connexion is displayed in 

 more various modes but with equal consistency. As before, 

 the Puff-ball served to exemplify the primary physiological 

 differentiation of outer parts from inner parts; so, here, it 

 supplies a simple illustration of the way in which the 

 differentiated outer part is re-differentiated, in correspon- 

 dence with the chief contrast in its relations to the environ- 

 ment. The only marked unlikeness which the cortical layer 

 of the Puff-ball presents, is that between the portion next 

 the ground and the opposite portion. The better-developed 

 Fungi exhibit a more decided heterogeneity of parallel kind. 

 Such incrusting Algae as Ralfsia verrucosa furnish a kin- 

 dred contrast; and in the higher Algce it is uniformly 

 repeated. Phsenogams display this physiologi- 



cal differentiation very conspicuously. That earth and air 

 are unlike portions of the environment, subjecting roots and 

 leaves to unlike physical forces, which entail on them unlike 

 reactions, and that the unlike functions and structures of 

 their respective surfaces are fitted to these unlike physical 

 forces, are familiar facts which it would be needless here 

 to name, were it not that they must be counted as coming 

 within a wider group of facts. 



Is this unlikeness between the outer tissues of the attached 

 ends and those of the free ends in plants, determined by 

 their converse with the unlike parts of the environment? 

 That they result from an equilibration partly arising in 

 the individual and partly arising by the survival of indivi- 

 duals in which it has been carried furthest, is inferable 

 a priori; and this a priori argument may be adequately 

 enforced by arguments of the inductive order. A few 

 typical ones must here suffice. The gemmules 



of the Marchantia are little disc-shaped masses of cells 

 composed of two or more layers. Their sides being alike, 

 there is nothing to determine which side falls lowermost 

 when one of them is detached. Whichever side falls lower- 

 most, however, presently begins to send out rootlets, while 



