MODERN VIEWS ON EVOLUTION 189 



to ' one type of organization '. The slighter differences 

 between them ' seem to lose themselves in the sameness 

 of form belonging to the genus, and even suggest a sus- 

 picion that they all proceeded from one original. The 

 phenomena of resemblance must have had their sufficient 

 reason as well as those of diversity '. He then inquires 

 whether the explanation is to be found in the action 

 of ' some slight modification in the productive causes ' 

 which stamped the genus at its first appearance ' with 

 all these specific diversities ' ; or whether, on the other 

 hand, a uniform genus was first created which 'after- 

 wards became diversified by the influence of external 

 agents'. He concludes that the former of these alterna- 

 tives is more strongly indicated by the knowledge of 

 his time. 



' Whichever of these suppositions may be true in point 

 of fact, the separation of families and genera into parti- 

 cular species, and the distribution of these species to 

 particular habitations, according to their physical proper- 

 ties, is evidently a part of the provision of nature for 

 replenishing the earth with organized inhabitants, placed 

 everywhere according to the congruity of soils and tem- 

 peratures, with their structure and habitudes. 



' But why is it to be supposed that the influence of this 

 law of adaptation has stopped here ? Is it not probable 

 that the varieties which spring up within the limits of 

 particular species, are further adaptations of structure to 

 the circumstances under which the tribe is destined 

 to exist [?] Varieties branch out from the common form 

 of a species, just as the forms of species deviate from the 

 common type of a genus. Why should the one class 

 of phenomena be without end or utility, a mere effect 

 of contingency or chance, more than the other ? 



' There are indeed many instances in which we can 

 perceive an advantage in the varieties of form, and an 

 adaptation of particular breeds to external circumstances.' 

 He then gives numerous examples — the small, active cattle 

 and horses which are found in mountainous countries, 

 the larger forms which flourish on fertile plains ; the 

 various breeds of the hog which are believed to hold 



