454 MORFBOLOGY. 



but in form. Consequently such parts, being already pres- 

 ent 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 orig- 

 inal 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 dis- 

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

 serial 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 indi- 

 vidual. 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. Kay Lankester, who has 

 drawn an important distinction between 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 de- 

 scent from a common progenitor with subsequent modifica- 

 tion, homogejious; 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 resem.blance of the parts on the right and left sides of 

 the body, and in the successive segments of the same indi- 

 vidual animal; and here we have parts commonly called 

 homologous which bear no relation to the descent of dis- 



