SUMMARY. 475 



On tliis same view of descent with modification, most 

 of the great facts in Morphology become intelligible — 

 whether we look to the same pattern displayed by the 

 different species of the same class in their homologous 

 organs, to whatever purpose applied; or to the serial and 

 lateral homologies 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 leading facts in embryology; namely, 

 the close resemblance in the individual embryo of the parts ^ 

 which are homologous, and which when matured become 

 widely different in structure and function; and the resem- 

 blance of the homologous parts or organs in allied though 

 distinct species, though fitted in the adult state for habUs 

 as different as is possible. Larvae are active embryos, 

 which have been specially modified in a greater or less 

 degree in relation to their habits of life, with their modifi- 

 cations inherited at a corresponding early age. On these 

 same principles, and bearing in mind that when organs 

 are reduced in size, either from disuse or through natural 

 selection, it will generally be at that period of life when 

 the being has to provide for its own wants, and bearing in 

 in mind how strong is the force of inheritance — the occur- 

 rence of rudimentary organs might even have been antici- 

 pated. The importance of embryological characters and 

 of rudimentary organs in classification is intelligible, on 

 the view that a natural arrangement must be genealogical. 



Finally, the several classes of facts which have been con- 

 sidered in this chapter, seem to me to proclaim so plainly, 

 that the innumerable species, genera and families, with 

 which this world is peopled, are all descended, each 

 within its own class or group, from common parents, and 

 have all been modified in the course of descent, that 1 

 should without hesitation adopt this view, even if it were 

 unsupported by other facts or arguments. 



