426 MORPHOLOGY 



many vertebrae ; the unknown progenitor of the Articulata, 

 many segments ; and the unknown progenitor of flowering 

 plants, many leaves arranged in one or more spires. We 

 .uave also formerly seen that parts many times repeated are 

 eminently liable to vary, not only in number, but in form. 

 Consequently such parts, being already present in consider- 

 able numbers, and being highly variable, would naturally 

 afford the materials for adaptation to the most different pur- 

 poses ; yet they would generally retain, through the force of 

 inheritance, plain traces of their original or fundamental 

 resemblance. They would retain this resemblance all the 

 more, as the variations, which afforded the basis for fcheir 

 subsequent modification through natural selection, would 

 tend from the first to be similar ; the parts being at an early 

 stage of growth alike, and being subjected to nearly the 

 same conditions. Such parts, whether more or less modified, 

 unless their common origin became wholly obscured, would 

 be serially homologous. 



In the great class of mollusks, though the parts in dis- 

 tinct species can be shown to be homologous, only a few 

 aerial homologies, such as the valves of Chitons, can be 

 indicated ; that is, we are seldom enabled to say that one 

 •part is homologous with another part in the same individual. 

 A.nd we can understand this fact; for in mollusks, even in 

 •the lowest members of the class, we do not find nearly so 

 much indefinite repetition of any one part as we find in the 

 other great classes of the animal and vegetable kingdoms. 



But morphology is a much more complex subject than it 

 At first appears, as has lately been well shown in a remark- 

 able paper by Mr. E. Bay Lankester, who has drawn an im- 

 portant distinction between certain classes of cases which 

 uave 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 modification, 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. 

 JVTr. Lankester also adduces the close resemblance of the 

 parts on the right and left sides of the body, and in the suc- 

 cessive segments of the same individual animal \ and here 



