SUMMARY 411 



borne in mind that the element of descent has been 

 universally used in ranking- together the sexes, ages, 

 and acknowledged varieties of the same species, however 

 different they may be in structure. If we extend 

 the use of this element of descent, — the only certainly 

 known cause of similarity in organic beings, — we shall 

 understand what is meant by the natural system : 

 it is genealogical in its attempted arrangement, with 

 the grades of acquired difference marked by the 

 terms varieties, species, genera, families, orders, and 

 classes. 



On this same view of descent with modification, all 

 the great facts in Morphology become intelligible, — 

 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 

 uecessarily 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 indi- 

 vidual embryo of the homologous parts, which when 

 matured 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 havo been even anticipated. 



