i4o CLUES AND SUGGESTIONS CHAP. 



an agency having a definite object, and 

 pursuing that object with a persistency 

 impossible to man as a mere breeder 

 of temporary varieties. This is an 

 argument which gives a very high rank 

 to species in the history of life. It is 

 because of it that Cuvier declared that 

 no science of Natural History is possible 

 if species be not stable. If, then, it be 

 true that one species has always given 

 birth to others, it must have been by 

 a process of which, as yet, we know 

 nothing. 



And then it must be remembered 

 that there are some fundamental features 

 in all living organisms involving corre- 

 sponding likenesses which can have no 

 other than a mental explanation. One 

 great principle governs the whole of 

 them, namely this, that in order to take 

 advantage of special laws, physical, 

 mechanical, chemical, and vital, certain 



