VARIATION AND DIFFERENTIATION. 19 



(6) When the proportionate size is considered, the order of compara- 

 tive variability of the legs is i, in, and n, whereas when we deal with 

 absolute size the order is I, II, and in, as shown in table 5 above. The 

 difference between legs n and in in mean variability is relatively greater 

 for the proportionate than for the absolute dimensions. 



(c) The great excess in variability indicated by the mean of the "leg i" 

 column of the table is, as before, clearly due in large measure to the extreme 

 variability of a single joint, the great chela. However we measure this 

 joint it is far and away the most variable of the lot. 



(d) The extremely low variability of all the appendage-cephalothorax 

 indices (with the exception of that involving the propodite of leg i) is 

 noteworthy. It is another expression of the fact of extremely high 

 correlation in the parts of the crayfish body, which we have already 

 seen. Schuster (loc. cit.) found a very low variability of the chela index 

 in Eupagurus. 



Putting all our results together, we have seen that when we compare 

 the several joints of three differentiated but serially homologous append- 

 ages of the crayfish in respect to relative variability, it is found that the 

 leg bearing the great chela is, in all the joints studied, the most variable. 

 This is true whether we deal with the variation of the parts in their 

 absolute sizes or with the variations in their proportionate dimensions 

 referred to the cephalothorax length as a base. But it is obvious that 

 the leg bearing the great chela is the one of those studied which is most 

 widely differentiated from the primitive type of the decapod limb, and 

 is also most highly specialized for the performance of a particular set of 

 functions. We hence conclude that, in the present case at least, a rela- 

 tively high degree of morphological differentiation and specialization 

 has associated with it a relatively high degree of variability in the parts 

 concerned. This, so far, is merely a statement of a fact regarding the 

 external morphology of the crayfish, and involves no theory as to the 

 nature or cause of such a relationship. An obvious suggestion as to the 

 cause of the greater variation shown by leg i is that since this is the 

 appendage which is on the whole most liable to injury, it will most often 

 be in process of regeneration. It might perhaps be maintained that our 

 sample contained a considerable number of individuals regenerating in 

 this leg, and that the greater variation really arose because of the inclu- 

 sion of different regeneration stages. It is possible that a part of the 

 observed result is to be explained in this way, but in our opinion the 

 influence of the possible inclusion of regenerating individuals must be 



