IV TENACITY OF HEREDITY 83 



But ill addition to tliis an important considuration lias 

 been entirely left out of consideration in Njigeli'.s ai\anneiit— 

 the length of time necessary for the production of important 

 modifications. 



When the external change favours completely the process 

 of evolution in progress, then the modification may— apart 

 from the possibility of the aid of adaptation— Ije brouglit 

 about very rapidly. In most cases, on the contrary, (jvun 

 when the external change is perfectly natural, the result must 

 evidently be very gradual. Leaving aside the very gradual 

 disappearance of unused organs, uj. parts of the skeleton, this 

 is proved by the obstinacy with which, as I have pointed out, 

 traces of primitive markings are inherited in animals. 



These facts prove the extreme tenacity of heredity. In 

 other words, they show how difficult it is to divert forms 

 from their usual direction of evolution, to conquer their vis 

 inertia:, and with what difficulty they can in general be led 

 into new paths of evolution. 



And this whole argument of mine shows also, to recur 

 again to the sul^ject of the previous sectiou, how unjustifiable 

 it necessarily is to talk of universal adaptation, for this very 

 tenacity of heredity is sufficient surety for the existence in 

 animals and plants of innumerable inessential characters 

 which have gone out of use or which belong to a deserted 

 path of evolution. 



Even natural science has found it hard to recognise the 

 effects of long periods of time. Only a few decades back 

 the 5000 years of the Bible were accepted even in 

 Geology. Now Geology reckons merely in the history of 

 living beings with endless time, and Darwin required a long 

 period of time to explain the modification of a single form. 

 But that the conviction is even at the present time far i'rom 

 forming part of the mental constitution of the naturalist, is 

 shown by the fact that so eminent and talented an iiKiuirer 



