9O Permanence and Evohition. 



from what we know of the effects of reversion, 

 we are likely to think that is the cause, unless we 

 know to the contrary. 



Very often we are able to trace particular 

 strains back for some time, as in the case of 

 the two branches of Herefords, which descend 

 from two cows belonging to Tomkins (Low, 

 " Domestic Animals," p. 364). I think there 

 can be no doubt that in proportion as we in- 

 vestigate animal forms, both wild and domestic, 

 so we derive an increased notion of the per- 

 manence of organised types. If we begin with 

 the popular notion of species, we come to see 

 that the principal varieties must be reckoned 

 absolutely permanent; then we see that this 

 holds equally of less marked sub-races ; further, 

 we perceive that we can only rest in still more 

 finely differentiated strains little, if at all, dis- 

 tinguished from what we generally call families, 

 till at last we are brought to recognise the 

 possibility of an idea which has been suggested, 



