ORGANIC GEOLOGICAL DYNAMICS. 56n 



tible, though difficult to define iu words, are limited in fact. It is 

 extremely interesting and satisfactory thus to receive an answer in 

 which we can confide, to inquiries seemingly so wide and bold as those 

 which this subject involves. I refer to Mr. Lyell, Dr. Prichard, Mr. 

 Lawrence, and others, for the history of the discussion, and for the 

 grounds of the decision ; and I shall quote very briefly the main points 

 and conclusions to which the inquiry has led. 5 



It may be considered, then, as determined by the over-balance of 

 physiological authority, that there is a capacity in all species to accom- 

 modate themselves, to a certain extent, to a change of external circum- 

 stances ; this extent varying greatly according to the species. There 

 may thus arise changes of appearance or structure, and some of these 

 changes are transmissible to the offspring : but the mutations thus super- 

 induced are governed by constant laws, and confined within certain 

 limits. Indefinite divergence from the original type is not possible ; 

 and the extreme limit of possible variation may usually be reached in a 

 brief period of time : in short, species have a real existence in nature, 

 and a transmutation from one to another does not exist. 



Thus, for example, Cuvier remarks, that notwithstanding all the 

 differences of size, appearance, and habits, which we find iu the dogs 

 of various races and countries, and though we have (in the Egyptian 

 mummies) skeletons of this animal as it existed three thousand years 

 ago, the relation of the bones to each other remains essentially the 

 same ; and, with all the varieties of their shape 6 and size, there are 

 characters which resist all the influences both of external nature, 

 of human intercourse, and of time. 



Sect. 4. Hypothesis of Progressive Tendencies. 



'\Vrraix certain limits, however, as we have said, external circumstances 

 produce changes in the forms of organized beings. The causes of 

 change, and the laws and limits of their effects, as they obtain in the 

 existing state of the organic creation, are'in the highest degree interest- 

 ing. And, as has been already intimated, the knowledge thus obtain- 



j 



ed, has been applied with a view to explain the origin of the existing 

 population of the world, and the succession of its past conditions. But 

 those who have attempted such an explanation, have found it necessary 

 to assume certain additional laws, in order to enable themselves to do- 



Lyell, B. iii. c. iv. Ossem. Foss. Disc. PreL p 61. 



