VARIATION AND CORRELATION IN THE CRAYFISH. WITH 

 SPECIAL REFERENCE TO THE INFLUENCE OF DIFFERENTIA- 

 TION AND HOMOLOGY OF PARTS. 



By Raymond Pearl and A. B. Clawson. 



INTRODUCTION. 



The general purpose with which this study was undertaken was to 

 investigate by biometrical analysis the laws of variation in a number of 

 organs which, while serially homologous, were at the same time differ- 

 entiated among themselves in varying degrees. Such systems of organs 

 are of course well known. A very clear example is afforded by the 

 appendages of a crustacean, say a decapod. In such an organism there is 

 strict serial homology between the segments of the different appendages. 

 At the same time, a given segment of any single appendage is distinctly 

 differentiated from the homologous segments of the others. The primary 

 and immediate aim of the present study was to obtain as definite answers 

 as possible to the following two questions: 



(i) What relation exists between the relative degree of variation 

 exhibited by any particular organ or character and its degree of differen- 

 tiation or specialization, as compared with other similar organs in a 

 homologous series? 



(ii) What relation exists between degree of correlation on the one 

 hand, and, on the other hand, the similarity of structure and position 

 indicated in the homology of parts in such a series of organs as is pre- 

 sented, for example, by the segments of the crustacean appendage? 



Naturally the data collected to answer these questions threw light on 

 other problems of variation, which will be considered further on in the 

 paper. Both of the main problems we had before us have been incident- 

 ally considered in several cases by earlier workers, but, so far as we are 

 able to learn, no systematic investigation, aimed at their elucidation and 

 working by quantitative methods on a considerable number of characters 

 of an organism, has ever been made. It seemed desirable that such an 

 investigation should be undertaken. 



1 



