332 SUMMARY AND CONCLUSIONS. 



There are evidently two ways in which a higher grade of symmetry 

 may be attained in these crystals, first by twinning, producing hexagonal 

 and tetragonal crystals from orthorhombic or monoclinic crystals, the 

 hexagonal structure requiring at least three individuals forming a triplet, 

 and generally having a very much larger number in the same orientation 

 as the three of the triplet; the tetragonal structure requiring a doublet, 

 and generally having a very much larger number of individuals, as in the 

 case of the hexagonal structure. The second way in which a higher grade 

 of symmetry is attained is by application of external pressures which neu- 

 tralize the internal tensions and produce isometric or sometimes tetragonal 

 structure. But when the mimetic crystal finally acquires a structure in 

 which the twin lamellae are ultra-microscopic, or the structure is so involved 

 by the plates not being continuous planes through the crystal that complete 

 averaging of the asymmetries takes place, it would seem possible that the 

 structure really changes to that of the higher grade of symmetry. That this 

 may take place in the molecule itself by change from one isomer to another 

 is rendered probable by the observation that the apparently pseudosym- 

 metric modification may develop de novo from the solution from which the 

 more unsymmetrical modification has been crystallizing, and without the 

 formation of any intermediate forms that are obviously twinned. This is 

 more likely to occur in chemical isomers than in physical isomers, and 

 would indicate that the apparently pseudosymmetric modifications were in 

 many cases chemical isomers. In large molecules like those of the hemo- 

 globins, plasticity of the molecule is very likely ; moreover, there is no doubt 

 from the recorded observations of the practical plasticity of the crystal 

 structure. 



The crystals of the hemoglobins form a general isomorphous series in 

 which the members are related to each other by simple ratios. They may 

 crystallize in any crystal system. Their elementary compositions may be 

 various or they may be stereoisomers of the same centesimal composition, 

 but all are connected by the common nucleus hemin, whose crystals show 

 angles that belong in the same isomorphous series. By mimesie a higher 

 grade of symmetry is attained, and perhaps this apparently mimetic sub- 

 stance may be an isomer of a parent substance. 



THE ZOOLOGICAL APPLICATIONS OF THIS METHOD OF RESEARCH. 



This method of studying the hemoglobins of various species of animals 

 furnishes the systematic zoologist a means of testing his findings in regard 

 to the relationships of different species and genera, and should prove a 

 valuable adjunct to the morphological data upon which he relies for estab- 

 lishing such relationships. The crystallographic characters that have been 

 recorded in the descriptions of the hemoglobins of species, and especially 

 the tabulations of them for the different groups of animals studied in this 

 research, show, as has been pointed out, that the characters of the crystals 

 of a genus are specific for each genus. The comparison of the crystals of 

 related species or genera show how closely they may approach each other 

 as regards this character. 



