1895.] Remarks on Variation in Animals and Plants. 379 



Evidently, therefore, in spite of the abnormally great frequency 

 with which large deviations occur, the whole percentage of abnor- 

 malities, among crabs between 7 and 14 mm. in length, is less than 

 it is in adult crabs; and there is a rough agreement between the 

 result obtained from these measurements and that obtained by 

 Bowditch from the measurements of human stature already referred 

 to. So that among female crabs in Plymouth Sound, during the 

 period of life to which these observations refer, there is no indication 

 of any destructive agency which acts selectively upon the dentary 

 margin. Whether such selective destruction occurs among males, or 

 among females at a later period of life, is for the present an open 

 question. 



Variation in frontal breadth may, therefore, for the present bo 

 considered to be of more importance in the economy of female crabs 

 than variation in the length of the dentary margin a view which 

 receives confirmation from the dimorphism already shown to exist 

 *(' Hoy. Soc. Proc.,' vol. 54, p. 324) in the frontal breadth of crabs 

 from Naples, while it is a striking justification of the accepted 

 system of classification, in which the characters of the great groups 

 into which the Brachyura are divided are almost entirely those 

 associated with changes in this dimension. 



In conclusion, an important feature of the method employed may 

 be pointed out. The increase of death-rate, associated with a given 

 abnormality of frontal breadth, has here been roughly determined ; in 

 the previous paper, already referred to, the effect of abnormality in 

 this dimension upon several other organs of the body was deter- 

 mined ; and by the method of that paper it would be possible to 

 determine the effect of parental abnormality upon the offspring. 

 These are all the data which are necessary, in order to determine the 

 direction and rate of evolution ; and they may be obtained without 

 introducing any theory of the physiological function of the organs 

 investigated. The advantage of eliminating from the problem of 

 evolution ideas which must often, from the nature of the case, rest 

 chiefly upon guess-work, need hardly be insisted upon. 



II. " Remarks on Variation in Animals and Plants. To accom- 

 pany the first Report of the Committee for conducting 

 Statistical Inquiries into the Measurable Characteristics of 

 Plants and Animals." By Professor W. F. R. WELDON, 

 F.R.S, Received February 19, 1895. 



1. The importance of variation as a factor in organic evolution is 

 not seriously disputed ; but, if one may judge from the expressions 

 contained in recent essays, naturalists are not agreed as to the 



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