HARRIS : CRAYFISHES GENUS OAMBARUS. 55 



lution of greater problems. A knowledge of the habits or dis- 

 tribution of the species of a genus is not in itself of the greatest 

 importance, but, if it enable us to determine something of the 

 nature of structural adaptations to environment, it is contributing 

 to the data for the solution of some of the problems of fundamen- 

 tal importance. The recognition of the fact that certain work is 

 preliminary in nature, and^that it is merely to furnish data for 

 work of greater importance which would be incomplete or im- 

 possible without it, is not the slightest justification for the neg- 

 lect of any field. Ecological questions are not the only ones, but 

 a thorough knowledge of the ecological relations of the different 

 species of a genus, in connection with a study of structural dif- 

 ferences and a consideration of physiographic changes affecting 

 distribution, will certainly contribute very greatly to a definite 

 knowledge of some phases of evolution. 



For several reasons Cambarus offers a particularly inviting 

 field for ecological work. A more thorough knowledge of the 

 habits of the animals may, possibly, contribute much to a full 

 understanding of the very remarkable sexual dimorphism occur- 

 ring in the adult males. Individual variation is very great and 

 puzzling — so much so that Professor Stimpson is said to have 

 remarked that either there is only one species of crayfish in 

 eastern North America or each mud puddle has its own. The 

 genus has certainly undergone a remarkable differentiation into 

 species. There are now recognized nearly eighty species and 

 varieties from the range of distribution in North America, as 

 compared with about thirteen species of Astacus, the other genus 

 of the crayfishes of the northern hemisphere, from Europe, 

 Asia, and America. As Faxon has pointed out, this difference 

 is not to be attributed to more thorough exploration of the 

 American territory. The chances for addition of species by 

 further exploration seem to be greatly in favor of Cambarus 

 rather than Astacus. Doctor Faxon ( '90) has said : " I have reason 

 to believe that oftentimes in this genus mere varieties, coming 

 into contact in a given locality, are perpetuated by breeding 

 true, when, by extending our geographical range, every inter- 

 mediate condition connecting the two forms will be found still 

 surviving. Nowhere do we seem to come so near to seeing the 

 process of evolution of species going on under our very eyes as in 

 this genus Cambarus. It seems to me that the only criteria of a 



