352 SPARKS FROM A GEOLOGIST'S HAMMER. 



nomena of organic life. Such a nisus was appealed to 

 by Lamarck; and Professor Huxley has more than once 

 hinted the probability that it is a potent factor in vital 

 phenomena. But it will be remarked that the admission 

 of such a nisus is not a final explanation. An inherent 

 nisus is causal, but not ultimately causal, unless we can 

 attribute to it all the characteristics which manifestly 

 belong to the efficiency which produces results in the 

 organism. 



When the question of fact has been answered ; when 

 the conditions under which the fact arises have been ascer- 

 tained, and the physical or physiological actions in the 

 organism which, under the conditions, work out the fact 

 have also been determined, this is as far as natural science 

 can go. The pure scientist may not care to extend his 

 inquiries farther. But assuredly this is not the ultimate 

 limit of rational inquiry. The human mind, in the com- 

 plete scope of its symmetrical activity, discerns other ques- 

 tions and makes further demands. Let us glance at 

 them. 



II. What are the Efficient Causes of Variative Deri- 

 vation of Species? 



Plainly, it may become shown that the mode of activity 

 of the organism, either conditioned or unconditioned by 

 the environment, is the means through .which the vital 

 phenomena of the world are brought to pass, and we may 

 still be ignorant of the efficient cause of that activity, or 

 of the subject exerting the activity. Now, even though 

 an indwelling and persistent nisus should appear to be 

 the principal impulse to physiological activities, we have 

 to seek after the source of the impulse. Does it originate 

 in the tissue in which it acts'? Is it a product of the 



