380 Prof. W. F. R. Weldon. [Feb. 28, 



manner in which variation among individuals is associated with 

 specific modification. 



The view originally put forward by Darwin and Wallace is that 

 specific modification is at least generally a gradual process, result- 

 ing from " the accumulation of innumerable slight variations, each 

 good for the original possessor" ('Origin of Species,' chap. xv). 

 This view rests on the assumption that each of those small differ- 

 ences which are to be observed among a group of individuals be- 

 longing to the same species has generally some effect upon the chance 

 of life. " Can we doubt (remembering that many more individuals 

 are born than can possibly survive) that individuals having any 

 advantage, however slight, over others, would have the best chance of 

 surviving and of procreating their kind ? " (' Origin of Species,' 

 chap. iv). 



Of late years, another view has received support from various 

 writers. An examination of any series of animals of the same 

 species preserved in a museum, shows in most cases a large majority- 

 of specimens which are superficially alike : those individual differ- 

 ences, upon which stress is laid by Darwin and by Wallace, are often 

 so slight as to escape attention unless minute comparison is made 

 between individual and individual. But there will commonly be 

 found a few individuals which differ so remarkably from their fellows 

 as to catch the eye at once. Such large deviations differ from the 

 smaller ones, at least in most cases, by their extreme rarity; but 

 they have been extensively collected, and most museums contain 

 numerous examples of their occurrence. Some naturalists have been 

 led, from the striking character of such variations, to assume for 

 them a preponderant share in the modification of specific character. 

 These persons assume, if I understand them rightly, that the advan- 

 tages or disadvantages which accompany the more frequent slight 

 abnormalities are in themselves of necessity slight; and that the 

 effect of such slight abnormalities may be neglected, in comparison 

 with the effect produced by the occasional appearance of considerable 

 deviations from the normal type. They regard change in specific 

 character as an event which occurs, not slowly and continuously, but 

 occasionally and by steps of considerable magnitude, as a consequence 

 of the capricious appearance of "sports." 



Without presuming to deny the possible effect of occasional 

 " sports " in exceptional cases, it is the object of the present remarks 

 to discuss the effect of small variations, as it may be deduced from 

 the study of two organs in a single species. 



The case chosen is the variation, during growth and in adult life, 

 of two dimensions of female Carcinus moenas, recently investigated 

 by a Committee of the Royal Society ; and what is here said may be 

 considered an appendix to the report of that Committee. 



