HOMOLOGY. 



49 



other, and yet the nipping-claw on one side is always bigger 

 than, and differently shaped to, the nipping-claw on the 

 other. Again, other Crustaceans have certain appendages 

 on one side which do not exist at all upon the other side. 

 Many animals, again, have the internal organs of one side 

 of the body either quite rudimentary or completely atro- 

 phied — as occurs, for example, in the Snakes — and yet they 

 show no external indication of this want of symmetry, nor 

 can we assert that the two sides of the body have been ex- 

 posed to conditions in any way dissimilar. Lastly, it can- 

 not be shown that those animals which exhibit no lateral 

 symmetry, or in which this symmetry is masked, have been 

 exposed to conditions in any respect different to those 

 affecting the bilaterally symmetrical animals which accom- 

 pany them; nor can it be shown that such animals have 

 been exposed to one set of conditions on one side of their 

 body, and another set of conditions on the other side. 



Ho7nogeiiy ajid Ho?nopIasy. — To meet the requirements of 

 those who hold the doctrine of the '* evolution. " of all exist- 

 ing species of organisms from other different pre-existent 

 forms, Mir Ray Lankester has recently proposed to super- 

 sede the term " homology," and to substitute for it the two 

 terms " homogeny " and " homoplasy." On this view only 

 those organs in different animals are " homogenous " which 

 owe their resemblances to genetic community of origin ; or, 

 in other words, to their having had " a single representative 

 in a common ancestor." On the other hand, Mr Lankester 

 asserts that when " identical or nearly similar forces, or 

 environments, act on two or more parts of an organism 

 which are nearly or exactly alike,^ the resulting modifications 

 of the various parts will be exactly or nearly alike ; " and 

 further, that " if, instead of similar parts in the same organ- 

 ism, we supppse the same forces to act on parts in two 

 organisms, which parts ate exactly or nearly alike* and 

 sometimes homogenetic, the resulting correspondences called 

 * The italics arc the author's, net Mr Lankester's. 



