XVI.] 



THE LAWS OF VARIATION. 199 



flying animals : it must have originated in some spon- 

 taneous variation. 



Among formative functions it appears to be a law that The 

 the minutest structures are the least variable. Thus, the structuro 

 form of the leaves of any species of tree is in gf'^neral J^s^J|i^^|«'ist 

 tolerably constant, while the form of the entire tree is 

 much more variable ; and the form of organs appears to be Organs 

 much more variable than the structure of tissues. Very ^ndtissues. 

 little is yet known on this subject. Darwin has made a 

 vast collection of facts showing the variability of organs ; 

 but no one has yet made a similar microscopic examination 

 of various tissues, for the purpose of ascertaining the 

 degree and the limits of their variability. There is, how- 

 ever, a remarkable constancy of microscopic structure 

 throughout the same species of organisms : thus, a small 

 fragment of a tooth, even when in a fossil state, is often Teeth, 

 enough for the identification of a species ; ^ and the same 

 is true of shells, even when their forms are subject to Shells. 

 considerable variation.^ Dr. Beale makes the following 

 striking remark on the constancy of histological characters 

 throughout the same species, and their distinctness from 

 those of other species : — 



" The anatomical differences between corresponding tis- 

 sues of closely allied species are often so distinct that the 

 anatomist familiar with them could distinguish the one 

 from the other. For example, it would be difficult to state 

 in few words the difference between the unstriped muscular Muscular 

 fibres of the bladder of the hyla [or tree-frog], of the *''"'''• 

 common frog, and of the newt, and yet there is a recog- 

 nisable difference; and corresponding differences can be 

 demonstrated in other textures, if a comparison be care- 

 fully instituted." ^ 



The subject, however, needs systematic investigation. Investi- 

 The variability of external form is visible to the eye in f^^J^g'^ 



1 Carpenter's Comparative Physiology, p. 152. 



= My friend the Rev. John Grainger, of Dublia, informs me that the 

 structure of shells presents little that is characteristic of particular species, 

 but the sculpture, or external markings, often enables the species to be 

 determined from very small fragments. 



3 Beale's edition of Todd and Bowman's Physiology, p. 41. 



