PHYSIOLOGICAL INTEGRATION IN PLANTS. 297 



the lower plants, both in space and time. " The much 

 narrower del imitation in area of animals than plants," says 

 Sir J. Hooker, " and greater restriction of Faunas than 

 Floras, should lead us to anticipate that plant-types are, geo- 

 logically speaking, more ancient and permanent than the 

 higher animal types are, and so I believe them to be, and I 

 would extend the doctrine even to plants of highly complex 

 structure." " Those classes and orders which are the least 

 complex in organization are the most widely distributed." 



§ 286. Thus that which the general doctrine of evolution 

 leads us to anticipate, we find implied by the facts. The 

 physiological division of labour among parts, can go on only 

 in proportion to the mutual dependence of parts; and the 

 mutual dependence of parts can progress only as fast as there 

 arise structures by which the parts are efficiently combined, 

 and the mutual utilization of their actions made easy. 



To say definitely by what process is brought about this 

 co-ordination of functions which accompanies their specializa- 

 tion, is hardly practicable. Direct and indirect equilibration 

 doubtless co-operate in establishing it. We may see, for 

 example, that every increase of fitness for function produced 

 in the aerial part of a plant by light, as well as every increase 

 of fitness for function. produced in its imbedded part by the 

 direct action of the moist earth, must conduce to an increased 

 current of the liquid evaporated from the one and supplied 

 by the other — must serve, therefore, to aid the formation of 

 sap-channels in the ways already described; that is — must 

 serve to develop the structures through which mutual aid 

 of the parts is given: the additional differentiation tends 

 immediately to bring about the additional integration. Con- 

 trariwise, it is obvious that an inter-dependence such as we 

 see between the secretion of honey and the fertilization of 

 germs, or between the deposit of albumen in the cotyledons 

 of an embryo-plant and its subsequent striking root, is a 

 kind of integration in the actions of the individual or of the 



