334 Evolution and Adaptation 



Since we are entirely in the dark as to how much time has 

 been required for the formation of phyla, so also are we 

 ignorant as to how long it may have taken for each step in 

 advance. We may err equally in ascribing too much and 

 too little time to the process. It is, moreover, not necessary 

 that for every step the same amount of time should have 

 been required. On the contrary, the probability is that 

 recognizable changes may at times follow each other rapidly, 

 and then for a time come to a standstill, — just as in the 

 development of the individual there are periods of more rapid 

 and others of less rapid change. 



A more difficult problem than that relating to the sort of 

 changes the external influences bring about in the organism, 

 is the question as to how they effect the organism, or how 

 they act on it mechanically. This, as is well known, was 

 answered by Darwin, who regards all organization as a prob- 

 lem of adaptation : only those chance variations surviving 

 which are capable of existence, the others being destroyed. 

 On this theory external influences have only a negative or a 

 passive action, namely, in setting aside the unadapted indi- 

 viduals. Nageli, on the other hand, looks upon some kinds 

 of external conditions as directly giving rise to the adaptive 

 characters of the organism. This is accomplished, he sup- 

 poses, in the following ways : two kinds of influence are 

 recognized ; the direct action, which, as in inorganic nature, 

 comes to an end when the external influences come to an 

 end, as when cold diminishes the chemical actions in the 

 plant ; and tJie indirect action, generally known as a stimulus, 

 which starts a series of molecular motions, invisible to us, 

 but which we recognize only in their effects. Very often 

 the stimulus starts only a reflex action, usually at the place 

 of application. 



A stimulus acting for but a short time produces no last- 

 ing effect on the idioplasm. A person stung by a wasp 

 suffers no permanent effect from the injury. But if a stim- 



