34 



MORPHOLOGICAL DEVELOPMENT. 



and become quite independent. From these transi- 



tional forms we pass, in the higher Jungermanniacece, to 



forms composed of many fronds that are permanently united 

 by a continuous stem. A more-developed aggregate of the 

 third order is thus produced. But though, along with in- 

 creased definiteness in the secondary aggregates, there is here 

 an integration of them so extensive and so regular, that they 

 are visibly subordinated to the whole they form; yet the 

 subordination is really very incomplete. In some instances, 

 as in Radula complanata, Fig. 47, the leaflets develop roots 

 from their under surfaces, just as the primitive frond does; 

 and in the majority of the group, as in J. capitata, Fig. 48, 

 roots are given off all along the connecting stem, at the spots 

 where the leaflets or frondlets join it: the result being that 

 though the connected frondlets form a physical whole, they 

 do not form, in any decided manner, a physiological whole; 

 since successive portions, of the united series, carry on their 

 functions independently of the rest. Finally, the 



most developed members of the group, whether lineally de- 

 scended from the less developed or from an early type com- 

 mon to the two, present us with tertiary aggregates which are 

 physiologically as well as physically integrated.* Not lying 



* The great mass of early ancestral types plant and animal consisting of 

 soft tissues, have left no remains whatever, and we have no reason to suppose 

 that those which left remains fell within the direct ancestral lines of any 

 existing forms. Contrariwise, we have reason to suppose that they fell with- 

 in lines of evolution out of which the lines ending in existing forms diverged. 

 We must therefore infer that the difficulties of affiliation which arise if we 



