280 PHYSIOLOGICAL DEVELOPMENT. 



of structure and habit and divisibility of groups among 

 the higher plants, appear relativel} 1 - marked. Though 

 Phaenogams are much more variable than most botanists have 

 until recently allowed, yet the definitions of species and 

 genera may be made with far greater precision and are 

 far less capable of change than among Cryptogams. 

 And this comparative fixity of type, implying, as it 

 does, a closer combination of the component functions, we 

 see to be the accompaniment of the greater differentiation of 

 those functions and of the structures performing them. That 

 these characters are correlatives is further shown by the 

 fact that the higher plants are more restricted in their 

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

 much narrower delimitation in area of animals than plants," 

 says Dr. Hooker, "and greater restriction of Faunas than 

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

 geologically 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 



