Chap. XIV. Morphology. 385 



plication of the parts developed from such cells. It must suffice for 

 our purpose to bear in mind that an indefinite repetition of the 

 same part or organ is the common characteristic, as Owen has 

 remarked, of all low or little specialised forms ; therefore the 

 unknown progenitor of the Vertebrata probably possessed many 

 vertebrae; the unknown progenitor of the Articulata, many seg- 

 ments; and the unknown progenitor of flowering plants, many 

 leaves arranged in one or more spires. We have 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 considerable numbers, and being highly 

 variable, would naturally afford the materials for adaptation to the 

 most different purposes ; 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 their 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 molluscs, though the parts in distinct 

 species can be shown to be homologous, only a few serial homo- 

 logies, 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. And we can understand this 

 fact ; for in molluscs, 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 remarkable paper by 

 Mr. E. Eay Lankester, who has drawn an important distinction 

 hetween certain classes of 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 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. Mr. Lankester also 



2 c 



