296 PHYSIOLOGICAL DEVELOPMENT. 



ability so great that what were at first taken to be different 

 species, or different genera, or even different orders, have 

 proved to be merely varieties of one species. So inconstant 

 in structure are the Algce that Schleiden quotes with approval 

 the opinion of Kutzing, that " there are no species but merely 

 forms of Algce:" an opinion which though now rejected 

 sufficiently implies extreme indefiniteness. In all which 

 facts we see that these lowest types of plants, little differ- 

 entiated, are also but little integrated. 



Archegoniates present a like relation between the small 

 specialization of functions which constitutes physiological 

 differentiation, and the small combination of functions which 

 constitutes physiological integration. " Mosses," says Mr. 

 Berkeley, " are no less variable than other cryptogams, and 

 are therefore frequently very difficult to distinguish. iSTot 

 •only will the same species exhibit great diversity in the size, 

 mode of branching, form and nervation of the leaves, but the 

 characters of even the peristome itself are not constant." 

 And concerning the classification of the remaining group, 

 Filicales, he says : — " Not only is there great difficulty in 

 arranging ferns satisfactorily, but it is even more difficult to 

 determine the limits of species." 



After this vagueness of separation as well as inconstancy 

 of structure and habit among the lower plants, the stability 

 of structure and habit and divisibility of groups among the 

 higher plants, appear relatively marked. Though Phaenogams 

 are much more variable than most botanists have until 

 lately allowed, yet the definitions of species and genera 

 may be made with far greater precision, and the forms 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 func- 

 tions 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 



