DOMAIN OF EVOLUTIONARY HYPOTHESIS 129 



The ' fertile ' leaves, which are united in special 

 clusters at the tips of the shoots (stalks) and hence 

 terminate the growth (Fig. 31), are very often differently 

 formed to the barren ones. 



(2) General results of systematic classification com- 

 pared with those of palceontology. 



From the above it is clear 

 that a priori we cannot know 

 whether our systematic cate- 

 gories are really primary and 

 do not signify forms of animals 

 and plants which are reducible 

 one from another, i.e. true 

 types in the sense of Cuvier. 

 Both elements which present 

 themselves in the definition of 

 a ' type ' plan of construction 

 (symmetry, habit as a whole), 

 and the degree of differentiation 

 in tissues and organs for the 

 general vital activities can in 

 themselves be subject to variation. It may therefore 

 happen that family and class characters may have arisen 

 out of other forms. 



It is therefore from the outset not excluded that, 

 for instance, all plants which have been assembled 

 under the classification of Ferns are only various modifi- 

 cations of one and the same form of growth and have 



FIG. 31. HABIT OF A CLTJB- 



MOSS (Lycopodium clavatum). 

 a, so-called 'bloom' = clus- 

 ter of spore vessels ; k, sepa- 

 rate sporangium burst open ; 

 s, leaflet (scale). 



