222 NATURAL SCIENCE [October 
was due to the selective action of an increase of silt in the water. 
Professor Weldon urged the necessity of extending as widely as 
possible this kind of numerical study. The difficulty of the theory 
of Natural Selection hes in the postulate that in any given case a 
small deviation from the mean character will be sufficiently useful 
or sufficiently harmful to affect the race. But determination of the 
deviation and of its effect on the death-rate is often possible. 
Whenever possible it is our duty to make it. “ Numerical know- 
ledge of this kind is the only ultimate test of the theory of Natural 
Selection, or of any other theory of any natural process whatever.” 
MorpHOLOGY 
PROFESSOR BOWER, in his address to the Section of Botany, placed 
before his hearers the principles of modern morphology, and dis- 
cussed the limits of their application. He advocated the establish- 
ment of classifications upon purely phylogenetic grounds. The 
attempt is beset with difficulties of all kinds, but it is the only 
goal of the taxonomist. Now this transference of our point of 
view from mere similarity of structure to questions of the origin 
of each structure brings into still greater relief the ever more 
complicated problems of homology. When we find that organs, 
structurally similar, have been independently developed in totally 
different races, how far can we consider them homologous? Ought 
we even to call them by the same names? The difficulties are 
manifest enough in every group of animals and plants; but often 
they are complicated by an alternation of generations, in which 
case the use of identical terms for organs that arise at absolutely 
different stages of life-history is apt to give rise to serious mis- 
conception. 
“Taking the case of leaves for the purpose of illustration, we 
may contemplate the following possibilities :—(a) A possible origin 
of two homoplastic series of leaves in the same plant, and the 
same generation (Phylloglossum) ; (b) Two homoplastic series in the 
same plant, but in different generations (Lycopodium cernuum); (ce) 
a possible distinct origin of homoplastic leaves in distinct phyla, but 
in the same generation (sporophyte of ferns, lycopods, equiseta) ; (d) 
a distinct origin of homoplastic leaves in distinct phyla, and distinct 
generations (eg. leaves of Bryophyta and of Pteridophyta). Now 
Homology has been used in an extended sense as including many, or 
even all, of these categories. It seems plain to me that this collec- 
tive use of the term homology carries no distinct evolutionary idea 
with it; it indicates little more than a vague similarity ; the word 
will have to be either more strictly defined or dropped. The old 
categories of parts based upon the place and mode of their origin 
are apt to be split up if the system be checked by views as 
