' 1885.] Pear Blight and its Cause. 1177 
socket shoulder joint capable of such varied and extensive motion, 
with a high degree of pronation and supination of the fore arm, 
and last, but not least, the wonderfully specialized hand with its 
thumb opposable to each of the four digits. 
After this brief survey, and with the forementioned proposition 
in view, viz., the correlation existing between the development of 
the clavicle and the work done by the fore limbs, we are left to 
draw the rational conclusion that the subject under consideration 
is one of use and disuse of parts, as Darwin has so clearly pointed 
out in his chapter on rudimentary organs in the Origin of Spe- 
cies. The facts we have noted in our hasty glance at the Mam- 
malia confirm this, in the more or less perfect development of 
clavicles in arboreal, fossorial, aérial and all other forms where 
the fore limbs are the active, aggressive pair in the life of the 
animal, and their absence or rudimentary condition in the hoofed 
animals, the marine species and all others where the anterior pair 
take a secondary place in the work done by the limbs. 
As there is, of course, no actual disuse of a part as a whole 
(the nearest approach to this being in marine forms), a simple, 
uncomplicated motion existing, with little strain at the shoulder 
joint, the parts require less support and fewer points for ligament- 
ous and muscular attachment than where the movements are 
more complicated and the strain more severe. Consequently we 
have a greater or less differentiation in the elements of the shoul- 
der girdle as the case may be, and the clavicle, holding as it does 
a position of secondary importance, is the unstable, variable ele- 
ment. 
A’. 
Ve 
PEAR BLIGHT AND ITS CAUSE. 
BY J. C. ARTHUR. 
EAR trees in this country are subject to an endemic disease 
that, owing to its malignancy and frequent occurrence, is well 
. known to cultivators and fairly well discriminated by them. It is 
known both as pear blight and fire blight, and the same disease in 
the apple and quince is also called twig blight. The term blight 
‘is applied to many kinds of plant diseases, and especially to those 
— that eventually kill without rendering the cause conspicuous; it 
is also the name of a class of disease-producing fungi. The pear 
malady bearing this name is, however, a specific disease, although 
