RECAPITULATION. 453 



buried beneath the great oceans. With respect to the lapse 

 of time not having been sufficient since our planet was con- 

 solidated for the assumed amount of organic change, and this 

 objection, as urged by Sir William Thompson, is probably 

 one of the gravest as yet advanced, I can only say, firstly, 

 that we do not know at w T hat rate species change, as meas- 

 ured by years, and secondly, that many philosophers are not 

 as yet willing to admit that we know enough of the consti- 

 tution of the universe and of the interior of our globe to 

 speculate with safety on its past duration. 



That the geological record is imperfect, all will admit; 

 but that it is imperfect to the degree required by our theory, 

 few will be inclined to admit. If we look to long enough 

 intervals of time, geology plainly declares that species have 

 all changed ; and they have changed in the manner required 

 by the theory, for they have changed slowly and in a gradu- 

 ated 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 sepa- 

 rated formations. 



Such is the sum of the several chief objections and 

 difficulties which may be justly urged against the theo^ ; 

 and I have now briefly recapitulated the answers and ex- 

 planations 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 

 which we are confessedly ignorant; nor do we know how 

 ignorant we are. We do not know all the possible transi- 

 tional 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 the}'" 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 varia- 

 tions as spontaneous. Variability is governed by many com- 

 plex laws, by correlated growth, compensation, the increased 

 use and Misuse of parts ? and the definite action of the sur- 



