ABSTRACT
Systematic characters, as currently used in analytical keys and definitions of taxa, are very diverse; some are evidently complex and intrinsically capable of subdivision, for example, the presence or absence of angiosperm-type carpels in vascular plants, or of pentadactyl limbs as against fins in Vertebrata. Correlations between particular pairs of characters may of course be attributable to functional connections—thus, for example, in animals, carnivorous adaptations of the mouth and alimentary canal tend to be correlated with adaptations of the sense-organs and locomotor apparatus, and among Dicotyledonae the positive correlation between the 'woody' habit and leaves with secretory cells is probably due to the functional correlation of both characters with long-persistent leaves. One thing which can be asserted on the basis of present-day knowledge is that it is rare for two distinct classificatory characters to be 100 per cent correlated in their incidence, and where two characters appear to show such a complete correlation, they are usually very closely related functionally.
