WHAT IS AN ORGANISM? l8l 



Paleocaridae ; it will have the twenty segments, the special- 

 ized carapace, the pair of mandibles, the two pairs of locked 

 maxillae, and other characters of the order Decapoda, and all 

 the peculiarities of the family Potamobiidae will be strictly 

 carried out. These concern the whole of the morphology, but 

 in some characters of still less importance the certainty is 

 not so great. This individual will develop on the first somite 

 or ring of the abdomen small appendages, certainly if it be 

 a male, and exceptionally if it be a female, whereas, if its 



FIG. 50. Astacus fluviatilis. Side view of a male specimen (nat. size), bg, branchiostegite; eg, 

 cervical groove ; r, rostrum; /, telson; i, eye-stalk ; 2, antennule ; 3, antenna; g, external 

 maxillipede ; 10, forceps; 14 last ambulatory leg; 17, third abdominal appendage ; xv, the 

 first and xx the last abdominal somite. (After Huxley.) 



ancestors had been the closely allied Parastacidae, no append- 

 ages would be developed. Again, in all the details of struc- 

 ture of parts it will be a true Astacus, and not a Cambarus, a 

 closely allied genus; and finally, if it were taken to California, 

 and placed under identical conditions with the native Astacus 

 nigrescens, it would still differ in all its specific characters from 

 that species characters which consist mainly in differences of 

 form and proportion of the parts, which are in number, 

 structure, and function the same for the two species. 



Slight Possible Effect of Environment. Environment might 

 produce slight modification in some of its very insignificant 

 characters, but otherwise rts total anatomy and physiology 



