Chap. XIII. SUMMARY. 457 



whether we look to the same pattern displayed in the 

 homologous organs, to whatever purpose applied, of the 

 different species of a class ; or to the homologous parts 

 constructed on the same pattern in each individual 

 animal and plant. 



On the principle of successive slight variations, not 

 necessarily or generally supervening at a very early 

 period of life, and being inherited at a corresponding 

 period, we can understand the great leading facts in 

 Embryology ; namely, the resemblance in an indivi- 

 dual embryo of the homologous parts, which when ma- 

 tured will become widely different from each other 

 in structure and function; and the resemblance in 

 different species of a class of the homologous parts or 

 organs, though fitted in the adult members for pur- 

 poses as different as possible. Larvae are active em- 

 bryos, which have become specially modified in relation 

 to their habits of life, through the principle of modifica- 

 tions being inherited at corresponding ages. On this 

 same principle — and bearing in mind, that when organs 

 are reduced in size, either from disuse or selection, it 

 will generally be at that period of life when the being 

 has to provide for its own wants, and bearing in mind 

 how strong is the principle of inheritance — the occur- 

 rence of rudimentary organs and their final abortion, 

 present to us no inexplicable difficulties ; on the con- 

 trary, their presence might have been even anticipated. 

 The importance of embryological characters and of 

 rudimentary organs in classification is intelligible, on 

 the view that an arrangement is only so far natural as 

 it is genealogical. 



Finally, the several classes of facts which have been 

 considered in this chapter, seem to me to proclaim 

 so plainly, that the inumerable species, genera, and 

 families of organic beings, with which this world is 



