MORPHOLOGY 247 



cases which have all been equally ranked by naturalists 

 as homologous. He proposes to call the structures which 

 resemble each other in distinct animals, owing to their 

 descent from a common progenitor with subsequent modi- 

 fication, homogenous; and the resemblances which cannot 

 thus be accounted for, he proposes to call homoplastic. 

 For instance, he believes that the hearts of birds and 

 mammals are as a whole homogenous — that is, have been 

 derived from a common progenitor; but that the four 

 cavities of the heart in the two classes are homoplastic 

 — that is, have been independently developed. Mr. 

 Lankester also adduces the close resemblance of the 

 parts on the right and left sides of the body, and ia 

 the successive segments of the same individual animal; 

 and here we have parts commonly called homologous, 

 which bear no relation to the descent of distinct species 

 from a common progenitor. Homoplastic structures are 

 the same with those which I have classed, though in a 

 very imperfect manner, as analogous modifications or 

 resemblances. Their formation may be attributed in part 

 to distinct organisms, or to distinct parts of the same 

 organism, having varied in an analogous manner; and in 

 part to similar modifications having been preserved for 

 the same general purpose or function — of which many 

 instances have been given. 



Naturalists frequently speak of the skull as formed 

 of metamorphosed vertebras; the jaws of crabs as meta- 

 morphosed legs; the stamens and pistils in flowers as 

 metamorphosed leaves; but it would in most cases be 

 more correct, as Professor Huxley has remarked, to speak 

 of both skull and vertebrie, jaws and legs, etc., as hav- 

 ing been metamorphosed, not one from the other, as they 



