244 
THE PLANT WORLD. 
at the utmost, bearing several leaves of the usual kind and ending 
in a hard pointed thorn-like extremity beyond them. This is the 
case, e . g., in Pyrus malifolia, P. communis, and Crataegus oxy- 
acantha. Again in Cotoneaster Pyracantha the acute side 
branch has functional foliage leaves, but they are much reduced 
in size, and the case approaches that of Crataegus Crus-galli. 
Finally, in the latter, the last essential step is taken; the leaves 
become non-functional, while the hardening and sharpening and 
in addition the heliotropic declination give the completed weapon 
of defence. 
The first definitely defensive modification probably affected 
only a small region of the extremity of the lateral branchlet, 
perhaps as a hardening of the axis of the winter bud. Reduc¬ 
tion of the leafage may have been the incidental result of a 
further increase of purely mechanical tissue and consequent de¬ 
crease of conducting elements of the wood, as the hardening ex¬ 
tended to the whole axis. But antecedent to every distinctively 
defensive modification was the critical first step—restriction of 
longitudinal growth. It is the most interesting aspect of this 
history—which, we must keep in mind, is plausible but not proven 
—that this step had relation not to the ultimate munitive function 
but to another use—leaf bearing. This is evolution by in¬ 
direction ; and it is characteristic of organic evolution that while, 
structurally considered, an organ may march straight from an 
initial to a final condition, the successive functions which make 
the several steps of this progress intelligible may form a cir¬ 
cuitous and unrelated series. This principle is to be borne in 
mind when the objection is raised against the theory of natural 
selection with gradual transformation, the objection that un¬ 
finished organs being useless the imperfect stages will not be 
preserved by selection and that therefore the evolution of these 
organs can not be attributed to any part played by them in the 
survival of the fittest; in short that evolving structures can not 
pay their way to the destination as, according to the theory, they 
should. But whether incipient transformations in any given case 
have been immediately and intrinsically useful can only be judged 
when the history of functions as well as of structures is known. 
