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 lies 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 (Phylloglosswm) ; (b) Two homoplastic series in the 

 same plant, but in different generations (Lycopodium cemuum) ; (c) 

 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 (e.g. 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 



