180 ON VARIABILITY AND ADAPTATION 



prismatic, but tending to have rounded edges and angles. If the 

 cover-glass be pressed (as I have repeatedly confirmed) they 

 become distorted, returning to their original shape when the pres- 

 sure is removed (Plate IV. Fig. 1). The latter under the ordinary 

 microscope show no signs of crystalline structure ; they appear 

 as masses of spherical bodies, capable of distortion, lying in a 

 singly refracting, isotropous matrix. But one and the same 

 substance may exhibit both forms ; there is no absolute division 

 into substances exhibiting purely the one or purely the other, 

 while if any of these bodies be distributed in an inert fluid matrix 

 the individual aggregations are peculiarly apt to take on the 

 sphero-crystalline form, appearing as doubly refracting globules 

 — identical with those that I have described to you as encountered 

 in the tissues. 



It need scarcely be said that these observations, completely 

 overturning the older ideas of the properties of crystals, have 

 from many quarters been regarded as heretical, and have en- 

 countered violent opposition. I am, as I have said, no physi- 

 cist, and should therefore not presume to weigh the evidence 

 that has been tendered against these conclusions. I can only 

 say that I have seen with mine own eyes that these sphero- 

 crystals and ductile crystals are capable of distortion (vide Plate 

 IV. Fig. 1) — that they are not solid. A very natural suggestion 

 has been made that we have here to deal with substances of two 

 orders, that, on heating, a purer more crystalline matter separates 

 out from a more inert fluid menstruum of different constitution. 

 G. Quincke, for example, has urged that the conditions corre- 

 spond with those found in emulsions, a fluid " skin " surrounding 

 and leading to the persistence of the separate globules. Tammann 

 has compared this stage to what is seen in an emulsion of carbolic 

 acid in water, which from being cloudy becomes transparent 

 on heating, and holds that here is not a true crystallization but 

 a depolarization phenomenon. Lehmann has brought forward 

 proof that we have to deal with true double refraction and not 

 a depolarization phenomenon, and he and Schenck have shown 

 that the phenomena present themselves equally well with 

 chemically pure substances of this order. 1 



1 Had we to deal with emulsions, centrifugating should separate the two 

 constituents, or, passing an electric current through the medium, the suspended 

 particles, if of different constitution, should gather at one or other pole. Schenck 

 has shown that with pure substances neither of these events occurs. 



