276 RECAPITULATION. [Chap. XV 



that species have all changed ; and they have changed 

 in the manner required by the theory, for they have 

 changed slowly and in a graduated manner. We clearly 

 see this in the fossil remains from consecutive formations 

 invariably being much more closely related to each other, 

 than are the fossils from widely separated formations. 



Such is the sum of the several chief objections and 

 difficulties which may be justly urged against the 

 theory; and I have now briefly recapitulated the 

 answers and explanations which, as far as I can see, 

 may be given. I have felt these difficulties far too 

 heavily during many years to doubt their weight. But 

 it deserves especial notice that the more important 

 objections relate to questions on winch we are con- 

 fessedly ignorant ; nor do we know how ignorant we 

 are. We do not know all the possible transitional 

 gradations between the simplest and the most perfect 

 organs ; it cannot be pretended that we know all the 

 varied means of Distribution during the long lapse of 

 years, or that we know how imperfect is the Geological 

 Record. Serious as these several objections are, in my 

 judgment they are by no means sufficient to overthrow 

 the theory of descent with subsequent modification. 



Now let us turn to the other side of the argument. 

 Under domestication we see much variability, caused, or 

 at least excited, by changed conditions of life ; but often 

 in so obscure a manner, that we are tempted to consider 

 the variations as spontaneous. Variability is governed 

 by many complex laws, — by correlated growth, compen- 

 sation, the increased use and disuse of parts, and the 

 definite action of the surrounding conditions. There is 

 much difficulty in ascertaining how largely our domestic 



