SPECIFICITY IN GENERIC AND SPECIFIC CHARACTERS. 327 



to the tabulated crystallographic characters of the hemoglobins of different 

 species given at the end of each chapter in the descriptions of species. 



Good examples of these variations of the crystals from the different 

 species of a genus may be seen by comparing the axial ratios of the crystals 

 of a-oxy hemoglobin of the Felidce or the crystals of reduced hemoglobin of 

 the same family, or the ratios of the crystals of /?-oxyhemoglobin of the 

 genus Papio. As already stated, the crystallographic characters of the 

 hemoglobin crystals seem to indicate that in case of the Canidoe the separa- 

 tion of the old genus Cam's into Cam's, Vulpes, and Urocyon is perhaps not 

 justified ; but in the separation of the wild cat, Lynx rufus, from the genus 

 Felis some support may be found in the characters of the crystals, although 

 they return to the normal form of the cats in the lynx, Lynx canadensis. 



The differences between the species are also shown in the habit of the 

 crystals, and this is dependent in part at least upon the differences in 

 solubility. The development of large crystals requires that the blood should 

 be moderately soluble, but when it is rather insoluble only small crystals 

 are liable to appear. Thus, in the four species of rats examined, two, 

 the Norway rat and the white rat, their oxyhemoglobins being insoluble, 

 normally produce only small crystals, while the black and Alexandrine 

 rats, having a more soluble oxyhemoglobin, show much larger crystals. 

 But the habit of certain planes developing in the crystals of some species 

 and not in those of others, and the different forms of growth of the crystal, 

 prismatic, tabular, etc., may produce very great differences in appearance 

 of the crystals, even when there may not be much difference in their angles. 



It is recognized that differences of crystal habit may depend upon 

 differences of composition as well as concentration of the solution from 

 which the crystals form, and, therefore, differences in substances that may 

 exist in the blood plasma of different species could influence the habit 

 of crystals in different species. In some cases the differences are mainly 

 in the way the crystals aggregate, and in the way they are related to each 

 other in the aggregates, and this would depend upon the rate of deposition 

 and upon the composition of the solution. In many cases two or more 

 habits of crystals of the same substance are recorded in one species, which 

 differences seem to depend upon concentration of the solution and the 

 pressure under which the crystals form. 



The specific character of the crystals from any species may perhaps 

 be best seen by referring to the photographic plates, where the above- 

 mentioned differences and similarities of the rats, for instance, are well 

 shown. The squirrels are another group in which the distinctions between 

 the crystals depending upon habit are quite marked, but the form and other 

 crystallographic characters are, of necessity, the same throughout the genus. 



The crystals obtained from different species of a genus are character- 

 istic of that species, but differ from those of other species of the genus in 

 angles or axial ratio, in optical characters, and especially in those characters 

 comprised under the general term of crystal habit, so that one species can 

 usually be distinguished from another by its hemoglobin crystals. But 

 these differences are not such as to preclude the crystals from all species of 

 a genus being placed in an isomorphous series. 



